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List T consist of 30 positive decimals, none of which is an [#permalink]
02 May 2012, 00:27

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Difficulty:

75% (hard)

Question Stats:

32% (02:59) correct
68% (02:10) wrong based on 463 sessions

List T consist of 30 positive decimals, none of which is an integer, and the sum of the 30 decimals is S. The estimated sum of the 30 decimals, E, is defined as follows. Each decimal in T whose tenths digit is even is rounded up to the nearest integer, and each decimal in T whose tenths digits is odd is rounded down to the nearest integer. If 1/3 of the decimals in T have a tenths digit that is even, which of the following is a possible value of E - S ?

I. -16 II. 6 III. 10

A. I only B. I and II only C. I and III only D. II and III only E. I, II, and III

Re: List T consist of 30 positive decimals, none of which is an [#permalink]
02 May 2012, 01:04

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First let us find the value of E: "1/3 of the decimals in T have a tenths digit that is even" => 10 numbers have an even tenths digit and remaining 20 numbers have an odd tenths digit. => E = (sum of all 30 integer parts) +10(1)-20(1) = (sum of all 30 integer parts)-10

Now let us come to "S": The maximum possible value of S occurs when ten numbers have '8' as tenths digit and remaining 20 numbers have '9' as tenths digit. Smax = (sum of all 30 integer parts) +10(0.8)+20(0.9) = (sum of all 30 integer parts)+26 The minimum possible value of S occurs when ten numbers have '2' as tenth digit and remaining 20 numbers have '1' as tenth digit. Smin = (sum of all 30 integer parts) +10(0.2)+20(0.1) = (sum of all 30 integer parts)+4

E-S ranges between (E-Smax) to (E-Smin) = -36 to -14

IMO, the answer option "A" is correct, as -16 only lies in this range. _________________

Re: List T consist of 30 positive decimals, none of which is an [#permalink]
02 May 2012, 04:07

vsrsankar wrote:

First let us find the value of E: "1/3 of the decimals in T have a tenths digit that is even" => 10 numbers have an even tenths digit and remaining 20 numbers have an odd tenths digit. => E = (sum of all 30 integer parts) +10(1)-20(1) = (sum of all 30 integer parts)-10

Now let us come to "S": The maximum possible value of S occurs when ten numbers have '8' as tenths digit and remaining 20 numbers have '9' as tenths digit. Smax = (sum of all 30 integer parts) +10(0.8)+20(0.9) = (sum of all 30 integer parts)+26 The minimum possible value of S occurs when ten numbers have '2' as tenth digit and remaining 20 numbers have '1' as tenth digit. Smin = (sum of all 30 integer parts) +10(0.2)+20(0.1) = (sum of all 30 integer parts)+4

E-S ranges between (E-Smax) to (E-Smin) = -36 to -14

IMO, the answer option "A" is correct, as -16 only lies in this range.

The answer I gave is the official guide answer. Plus I dont understand how you chose 0.8 and 0.9, can you elaborate on that? also the sum of E , shouldnt it be 10 ( x) +20(y) ....instead of minus as you indicated above. Perhaps I am missing something, and would appreciate further clarification.

Re: List T consist of 30 positive decimals, none of which is an [#permalink]
02 May 2012, 07:13

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shamanth25 wrote:

List T consist of 30 positive decimals, none of which is an integer, and the sum of the 30 decimals is S. The estimated sum of the 30 decimals, E, is defined as follows. Each decimal in T whose tenths digit is even is rounded up to the nearest integer, and each decimal in T whose tenths digits is odd is rounded down to the nearest integer. If 1/3 of the decimals in T have a tenths digit that is even, which of the following is a possible value of E - S ?

I. -16 II. 6 III. 10

A. I only B. I and II only C. I and III only D. II and III only E. I, II, and III

what is the best way to solve this question.

many thanks S

This is how I would solve it:

Even tenth digit - Round up - 10 numbers Odd tenth digit - Round down - 20 numbers

E - S can take many values so how do we figure which ones it cannot take? We need to find the range of E - S - the minimum value it can take and the maximum value it can take.

Minimum value of E - S => E is much less than S. How do we make E much less than S? By doing 2 things:

1. When I round up, the difference between actual and estimate should be little. Say the numbers are something like 3.8999999 (very close to 3.9) and they will be rounded up to 4 i.e. the estimate gains 0.1 per number. Since there are 10 even tenth digit numbers, the estimate will be apprx .1*10 = 1 more than actual 2. When I round down, the difference between actual and estimate should be huge. Say the numbers are something like 3.999999 (very close to 4) and they will be rounded down to 3 i.e. the estimate loses apprx 1 per number. Since there are 20 such numbers, the estimate is 1*20 = 20 less than actual. Overall, the estimate will be apprx 20 - 1 = 19 less than actual

E - S = -19

Maximum value of E - S => E is much greater than S. How do we make E much greater than S? By doing 2 things:

1. When we round up, the difference between actual and estimate should be very high. Say the numbers are something like 3.000001 (very close to 3) and they will be rounded up to 4 i.e. the estimate gains 1 per number. Since there are 10 even tenth digit numbers, the estimate will be apprx 1*10 = 10 more than actual 2. When we round down, the difference between actual and estimate should be very little. Say the numbers are 3.1. They will be rounded down to 3 i.e. the estimate loses apprx 0.1 per number. Since there are 20 such numbers, the estimate is 0.1*20 = 2 less than actual.

Maximum value of E - S = 10 - 2 = 8

So 10 cannot be the value of E - S. _________________

Although NOT most of GMAT Quantum's video explanations/solutions are up to the par, I found that this one along with some others (e.g., PS 178) is the best video explanation floating out there in the internet.

Although NOT most of GMAT Quantum's video explanations/solutions are up to the par, I found that this one along with some others (e.g., PS 178) is the best video explanation floating out there in the internet.

Re: List T consist of 30 positive decimals, none of which is an [#permalink]
30 Jul 2012, 23:11

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Expert's post

Assume T = (1.a, 1.b,...etc) All units equal 1.xx. E = 40 (due to rounding of ten even and 20 odd) S max = 30 + 10(.8) + 20(.9) = 56 S min = 30 + 10(.2) + 20(.1) = 34

E-S min = 40 - 56 = -16 E-S max = 40 - 34 = 6 Thus, the min/max of E is -16 and 6, so I , II apply.

For (III. 10) to be true, S min/max in our equation above needs to equal 30. This is impossible since the question states T consists of 30 positive decimals. Therefore eliminate III. _________________

Re: List T consist of 30 positive decimals, none of which is an [#permalink]
19 Dec 2012, 02:49

shamanth25 wrote:

vsrsankar wrote:

First let us find the value of E: "1/3 of the decimals in T have a tenths digit that is even" => 10 numbers have an even tenths digit and remaining 20 numbers have an odd tenths digit. => E = (sum of all 30 integer parts) +10(1)-20(1) = (sum of all 30 integer parts)-10

Now let us come to "S": The maximum possible value of S occurs when ten numbers have '8' as tenths digit and remaining 20 numbers have '9' as tenths digit. Smax = (sum of all 30 integer parts) +10(0.8)+20(0.9) = (sum of all 30 integer parts)+26 The minimum possible value of S occurs when ten numbers have '2' as tenth digit and remaining 20 numbers have '1' as tenth digit. Smin = (sum of all 30 integer parts) +10(0.2)+20(0.1) = (sum of all 30 integer parts)+4

E-S ranges between (E-Smax) to (E-Smin) = -36 to -14

IMO, the answer option "A" is correct, as -16 only lies in this range.

The answer I gave is the official guide answer. Plus I dont understand how you chose 0.8 and 0.9, can you elaborate on that? also the sum of E , shouldnt it be 10 ( x) +20(y) ....instead of minus as you indicated above. Perhaps I am missing something, and would appreciate further clarification.

thanks Shamanth

I am not sure whether you´ll still need this explanation of how the values of 0.8 an 0.9 account into the equation, but I had the same question after I realized that 0.8 is the highest even value the decimals can have (smallest even is 0.2) and 0.9 is the highest odd value the decimal can have (smallest odd is 0.1). By using these figures in the equation one will be able to compute for highest /lowest possible value At leat thats how I understood it

Re: List T consist of 30 positive decimals, none of which is an [#permalink]
23 Mar 2013, 12:00

souvik101990 wrote:

Assume T = (1.a, 1.b,...etc) All units equal 1.xx. E = 40 (due to rounding of ten even and 20 odd) S max = 30 + 10(.8) + 20(.9) = 56 S min = 30 + 10(.2) + 20(.1) = 34

E-S min = 40 - 56 = -16 E-S max = 40 - 34 = 6 Thus, the min/max of E is -16 and 6, so I , II apply.

For (III. 10) to be true, S min/max in our equation above needs to equal 30. This is impossible since the question states T consists of 30 positive decimals. Therefore eliminate III.

To clarify, when you got E = 40 1.xxx rounded up, 2 x 10 (even) = 20 1.xxx rounded down, 1 x 20 (odd) = 20

Re: List T consist of 30 positive decimals, none of which is an [#permalink]
15 Nov 2013, 03:27

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Ok, so I don't normally post here, but one of my private tutoring students pointed me to this thread.

So yeah, there are a lot of words in this problem, and it looks complicated. And if you actually start thinking about "E" and "S" separately -- as most people on this thread seem to be doing -- then it becomes quite complicated indeed. But ... we don't care about E, and we don't care about S. We care about E - S. That's a HUGE difference. (Analogy: I drive my car 54 miles. Do you know the starting odometer mileage? Nope. Do you know the ending odometer mileage? Nope. Do you know the difference between them? Yep, 54 miles.)

What affects E - S? To figure that out, temporarily forget that there are 30 numbers, and pretend there's just 1 number. * If the number is rounded up, then E - S is positive. It's the amount by which the number is rounded up. (If you round 14.87 up to 15, then E - S is 0.13.) * If the number is rounded down, then E - S is negative. It's the amount by which the number is rounded down. (If you round 14.77 down to 14, then E - S is -0.77.) Thinking this way, we can see that it's a waste of time to consider the "integer part" at all. If you round 14.87 up to 15, or 99.87 up to 100, or anything point 87 up to the next integer, you're still looking at +0.13.

Now that we know that, things are more transparent.

* 10 numbers are rounded up. The most by which they can be rounded up is just barely less than 1 each (if you start with x.00001 type thing). The least by which they can be rounded up is just barely more than 0.1 each (if you start with x.89999 type thing).

* 20 numbers are rounded down. The most by which they can be rounded down is just barely less than 1 each (if you start with x.99999 type thing). The least by which they can be rounded down is 0.1 each (if you start with x.1).

To make E - S as big as possible, make the positive contributions as big as possible: 10 * (approximately +1) = approximately +10. Make the negative contributions as small as possible: 20 * -0.1 = -2. So, the maximum value of E - S is somewhere around 8.

To make E - S as small as possible, make the positive contributions as small as possible: 10 * (approximately 0.1) = approximately +1. Make the negative contributions as big as possible: 20 * (approximately 1) = approximately -20. So, the minimum value of E - S is somewhere around -19.

There you go.

The point here is that you should think about whatever the problem actually asks you for. In those terms, it sounds stupidly simple, but look at what's happening in this thread -- everyone is thinking separately about E and S, even though we only care about E - S. (Think about the odometer.) Oops. Focus!

This idea is even more important on data sufficiency problems. (If a data sufficiency problem asks for something minus something else, and you try to find the individual values of "something" and "something else" rather than just finding the difference, you're practically certain to get the problem wrong, even if your math is all correct.)

--

Ron Purewal ManhattanGMAT

Un bon vêtement, c'est un passeport pour le bonheur. Yves Saint-Laurent

Re: List T consist of 30 positive decimals, none of which is an [#permalink]
15 Jan 2014, 17:39

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I found the responses in this question very helpful, but I think I found another way to explain it that I can understand better... hopefully someone finds this useful someday.

** As the question ask for the difference between S and E, the actual "integer" portions are negligible as they will cancel each other out in the minus (-) operation (e.g. 15.9 - 15 = 0.9) . If it helps in understanding, we can also assume that all numbers in the list T are 0.xxxx (as the minus operation cancels the "integer" portion anyway).

Let's recap: S = Real sum E = Estimated sum, 10 are rounded up, 20 are rounded down

We can also first establish that there is only one value of E, as we are given in the question that 10 of them WILL be rounded up, and 20 WILL be rounded down. There is no Emax or Emin. To solve for E and assuming that numbers in list T are 0.xxx as per above point, E = 10 x (T values rounded up) + 20 x (T values rounded down) = 10 x (0.2) + 20 x (0.9) --> doesnt matter what the values are, they can be any even / odd combinations at the tenth = 10 x (1) + 20 x (0), after performing round up/down operations = 10

There are however Smax and Smin, as these are possible ranges prior to the round up/down operations.

Smax = 10 x (maximum even tenth) + 20 x (maximum odd tenth) = 10 x (0.8) + 20 x (0.9) = 8 + 18 = 26

Smin = 10 x (minimum even tenth) + 20 x (minimum odd tenth) = 10 x (0.2) + 20 x (0.1) = 2 + 2 = 4

The largest range of E - S is therefore E - Smin = 10 - 4 = -6

The lowest range of E - S is therefore E - Smax = 10 - 26 = -16

Unlike some contributions here, I found it absolutely necessary to solve for E and S. The previous solution by RonPurewal looks sound, but when you look closer, it assumes that E - S = (10 round up values - 20 round down values), which is not true, and therefore the answers are not exactly to the range given by the question. The full algebraic equation of his solution should have been:

- Largest range of E - S = (10x round up + 20x round down) - (10x Minimum round up + 20x minimum round down) - Lowest range of E - S = (10x round up + 20x round down) - (10x Maximum round up + 20x maximum round down)

Essentially calling for the need to resolve for E and S individually, eventually. I apolgise if I sound rude - its 130am in the morning and I had just spent an hour just analyzing just this question alone, and my heart hurts! Irregardless, I have learn plenty just by analyzing everyone's solution to help me arrive to this way of looking into this problem. Many thanks!

Although NOT most of GMAT Quantum's video explanations/solutions are up to the par, I found that this one along with some others (e.g., PS 178) is the best video explanation floating out there in the internet.

it only explains the E i dont understand how he moved to S