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(1) only: during the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
We don't know

(2) only: during the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.
We don't know

Both (1) & (2): we don't know the number of boxes per period, so we don't know.

Answer: E
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Statement (1)
With this information we can't determine the overall AM. A and D are out.

Statement (2)
With this information we can't determine the overall AM. B is out.

Statement (1) and (2)
Taking both the statements we can arrive at the overall AM. C is correct.

Bunuel
Over a 4-hour shift, Mark loaded a total of 80 boxes into a truck. What was the average (arithmetic mean) weight of the boxes he loaded during the shift?

(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.


 


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As we dont know the number of boxes whose Avg is 20 and 30 we cannot calculate the Avg of the total.
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Rate = 80/4= 20 boxes per hour

(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.

average weight of the first 40 boxes = 20 kg.
We don't know anything about the weight during the 2nd half. Not sufficient

(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.

average weight of the second 40 boxes = 30 kg.
We don't know anything about the weight during the 1st half. Not sufficient

(1)+(2)

Total average = 40(20) + 40(30)/80= 25 Kg

Correct answer is (C)


Bunuel
Over a 4-hour shift, Mark loaded a total of 80 boxes into a truck. What was the average (arithmetic mean) weight of the boxes he loaded during the shift?

(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.


 


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Over a 4-hour shift, Mark loaded a total of 80 boxes into a truck.

What was the average (arithmetic mean) weight of the boxes he loaded during the shift?

(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
Total weight of boxes loaded during first 2 hours of the shift = 20x; x = number of boxes loaded during first 2 hours of the shift
Since there is no information about last 2 hours of the shift, average weight of the boxes he loaded during the shift can not be derived
NOT SUFFICIENT

(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.
Total weight of boxes loaded during last 2 hours of the shift = 30y; y = number of boxes loaded during last 2 hours of the shift
Since there is no information about first 2 hours of the shift, average weight of the boxes he loaded during the shift can not be derived
NOT SUFFICIENT

(1) + (2)
(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.
Total weight of boxes loaded during first 2 hours of the shift = 20x; x = number of boxes loaded during first 2 hours of the shift
Total weight of boxes loaded during last 2 hours of the shift = 30(80-x) = 2400 - 30x;
Total weight of boxes loaded during the 4 hours shift = 2400 - 30x + 20x = 2400 - 10x
Average weight of boxes loaded during the 4 hours shift = (2400 - 10x)/80 = 30 - x/8
Since x is unknown
NOT SUFFICIENT

IMO E
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Bunuel
Over a 4-hour shift, Mark loaded a total of 80 boxes into a truck. What was the average (arithmetic mean) weight of the boxes he loaded during the shift?

(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.


 


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Over a 4 hour shift, Mark loaded a total of 80 boxes.

Average weight of boxes = ?

Average weight of boxes = ( Total weight) / ( Total number of boxes) = ( Total Weight) / 80

Statement 1:

(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.

Let’s take the number of boxes that was loaded during first two hours as a.

Average Weight for first 2 hours = 20 = ( sum of weights loaded for first two hours)/ a

Sum of weights loaded for first two hours = 20*a

Hence, insufficient

Statement 2:

(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.

Let the number of boxes loaded during the last two hours = b

Average Weight for Last 2 hours = 30 = ( sum of weights loaded for last two hours ) / b

Sum of weights loaded for last two hours = 30*b

Hence, insufficient

Combining Statements 1 and Statements 2,

a + b = 80

Average weight = [ 20*a + 30* (80-a) ] / 80

= ( 20*a + 2400 - 30*a ) / 80

= ( 2400 - 10*a) /80

Hence, insufficient

Option E
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We need a sum of 80 boxes of weight
(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
But how many boxes and no information about the last 2 hr, not sufficient
(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.
But how many boxes and no information about First 2 hr, not sufficient

combined, First 2 hr X boxes, then last 2 hr = 80-x

Weight = 20*x + 30*(80-X )
= 2400 - 10X

Avg = (2400-10X)/80

After combining we are not getting value of x, then Ans E, both are not sufficient
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There are 80 boxes
Total weight of box = W
average weight = W/80 = ?

Statement 1:
During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
We need more info to calculate final weight. Number of boxes in first 2 hrs is unknown, what happened in next 2 hrs is also unknown.

Statement 2:
During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.
We need more info to calculate final weight. Number of boxes in last 2 hrs is unknown, what happened in first 2 hrs is also unknown.

Combining both 1&2:
Let number of boxes in first 2 hrs be x, number of boxes in last 2 hrs = 80-x
=> W = 20x+30(80-x) = 2400-10x

Here also we need more info to calculate W

Hence E is correct
Bunuel
Over a 4-hour shift, Mark loaded a total of 80 boxes into a truck. What was the average (arithmetic mean) weight of the boxes he loaded during the shift?

(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.


 


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average is based on two equal time periods which we know per the question stem. therefore, it is an equally weighted average, and we can determine the average by taking the average of the two averages, so C together both statements are sufficient.
Bunuel
Over a 4-hour shift, Mark loaded a total of 80 boxes into a truck. What was the average (arithmetic mean) weight of the boxes he loaded during the shift?

(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.


 


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given in 4 hrs. shift no of boxes loaded=80
avg. weight of boxes loaded in the shift=?
1: avg. weight of boxes loaded in first two hrs.=20 kg NON SUFF

2:avg. weight of boxes loaded in the last two hrs.=30 kg NON SUFF
together with law of averages its sufficient to find out the avg. weight loaded by mark in the 4 hrs. shift=(20*2+30*2)/4=25
hence C
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Statement 1:
This statement does not give enough information. We don't know how many he loaded in the first 2 hours, and we don't know anything about the weight of the boxes he moved in the last 2 hours.

Statement 2:
This statement does not give enough information. We don't know how many he loaded in the last 2 hours, and we don't know anything about the weight of the boxes he moved in the first 2 hours.

Both statements together:
Both statements together still are not enough information to calculate the total average weight. To calculate the average, we need to know the ratio of the number of boxes he loaded in each of the 2 hours. Without this information, we could have different answers. For example, if he loaded 10 and 70 boxes in each of the 2 hours, or if he loaded 70 and 10 in each of the 2 hours, it would give us different averages. So, both statements together are not enough.

The answer is E.

Bunuel
Over a 4-hour shift, Mark loaded a total of 80 boxes into a truck. What was the average (arithmetic mean) weight of the boxes he loaded during the shift?

(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.


 


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"Over a 4-hour shift, Mark loaded a total of 80 boxes into a truck. What was the average (arithmetic mean) weight of the boxes he loaded during the shift?"

Average Weight = SUM ( boxes weights ) / 80

Work formula (if the statements show us that it's constant )
R * T = W
R * 4 = 80
R = 80/4
R = 20

(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
Ok. But for now we do not know how many boxes were loaded in this 2 hours. It could be 40, if it was a constant rate, but could also be 0 or 79. Is it sufficient to answer? No, it's not. Eliminate answer choices A and D.


(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.
Ok. But for now we do not know how many boxes were loaded in this 2 hours. It could be 40, if it was a constant rate, but could also be 80 or 1. Is it sufficient to answer? No, it's not. Eliminate answer choice B.


Statements (1) and (2) combined
We do not have any information about a constante work rate or even the boxes amount in each time window. If we have, for instance, 10 boxes in (S1) and 70 boxes in (S2), we would arrive in a different average than if we have the amounts swapped. Therefore, eliminate answer choice C.

Answer = E Statements (1) and (2), together, are not sufficient to answer.
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Bunuel
Over a 4-hour shift, Mark loaded a total of 80 boxes into a truck. What was the average (arithmetic mean) weight of the boxes he loaded during the shift?

(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.


 


This question was provided by GMAT Club
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(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.

No information on the number of boxes loaded in the time frame provided and the number and weight of the boxes loaded in the remaining time frame. Hence, this statement alone is not sufficient.

(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.

Same as Statement 1. No information on the number of boxes loaded in the time frame provided and the number and weight of the boxes loaded in the remaining time frame. Hence, this statement alone is not sufficient.

Combined.

Now while we have the average weight, we don't know the split on the number of boxes loaded in each hour. Hence, we can't find the average (arithmetic mean) weight of the boxes he loaded during the shift

Option E
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Mark loaded 80 boxes in 4 hours. We have to find the avg weight of all 80 boxes.

Statement 1: During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
There is no info about how many boxes he loaded in that time, so we can’t find total weight
Insufficient

Statement 2: During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.
Again the same issue as Stmt. 1
Insufficient

even if we use both statements, we still don’t know how many boxes were loaded in first 2 hrs. vs. last 2 hrs.
So we can’t calculate the total weight of all 80 boxes and thus can't calculate the avg.

Answer: E
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Over a 4-hour shift, Mark loaded a total of 80 boxes into a truck. average (arithmetic mean) weight of the boxes during the shift = ?
total time, T = 4hours, B = 80 boxes,
Point to be noted that no weight is mentioned and additionally both statement contains weight information

Statement (1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
T first half, even if we consider as total weigh in first 2 hours = 20 (Number of boxes loaded in first 2 hours)
as we don't have count of "Number of boxes loaded in first 2 hours", we can say Statement (1) is not sufficient

Statement (2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.
T second half , even if we consider as total weigh in second 2 hours = 30 (Number of boxes loaded in second 2 hours)
as we don't have count of "Number of boxes loaded in second 2 hours", we can say Statement (2) is not sufficient

Now lets consider Statement (1) and Statement(2), as no info on average weights depending on how the 80 boxes are distributed between the first 2 hours and the last 2 hours, even with both statements combined, we cannot determine a single value for the average weight of all 80 boxes.
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Bunuel
Over a 4-hour shift, Mark loaded a total of 80 boxes into a truck. What was the average (arithmetic mean) weight of the boxes he loaded during the shift?

(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms.
(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms.


 


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1) Not sufficient alone, as we dont know about the quantity of boxes loaded.
2) Not sufficient alone, as we dont know about the quantity of boxes loaded.

Together unsufficient as well, as we still have no idea about the actual distribution of the boxes.
It would be different, if there would be a mention, that the rate of loading did not change over the course of the whole process, we would be able to get an exact result.
But in that case, that is not possible.
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We know number of boxes but we don't have their weights to find avg so lets see

(1) During the first 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 20 kilograms. => Ok with even when we know the avg 20 we don't know how many boxes mark loaded. so this is Not Sufficient

(2) During the last 2 hours of the shift, the average (arithmetic mean) weight of the boxes Mark loaded was 30 kilograms. => Again same in this 2 hours we don't know how many boxes mark loaded so this is also insufficient.

Combine 1 and 2 =>
lets say x loaded in first 2 hours
Then in last 2 hours 80-x loaded.
so total weight = 20x + 30(80-x)
here we still don't know the x so we can't find avg.
so Not Sufficient

Hence Ans E
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