In a particular machine, there are 2 gears that interlock; One gear is

Question

In a particular machine, there are 2 gears that interlock; One gear is larger in circumference than the other. The manufacturer of the gears guarantees that each gear will last for atleast 6,000,000,000 revolutions. Assuming that there is no slippage between the 2 gears and that when one gear rotates the other gear also rotates, the larger gear is guaranteed to last how many days longer than the smaller gear.

(1) The diameter of the larger gear is twice the diameter of smaller gear.
(2) The smaller gear revolves 600 times per minute.

(1) The diameter of the larger gear is twice the diameter of smaller gear.
(2) The smaller gear revolves 600 times per minute.

EvaJager · Accepted Answer

(1) We can deduce that the smaller gear makes two rotations while the larger gear makes just one rotation. No information about the speed of revolution, so we cannot establish any connection with time.
Not sufficient.

(2) No information about the larger gear.
Not sufficient.

(1) and (2): The larger gear makes 300 rotations per minute and the smaller gear 600 per minute. So, we can calculate how long it takes in minutes each one to do 6,000,000,000 rotations, take the difference and convert to days...really messy. But being a DS question, you shouldn't spend time on working out the final answer.

Choose Answer C, and move on.

PrashantPonde · Answer

Each gear has life to 6,000,000,000 revolutions. As both gears are interlocked, smaller gear will have more revolutions and will wear out before the larger one.

(1) The diameter of the larger gear is twice the diameter of smaller gear.
 INSUFFICIENT : This means - the circumference of the larger gear is twice that of smaller gear. I.e. 1 revolution of larger gear = smaller gear rotates two times. 
Even if we know relative ratio for diameters, we don’t know what rate at they are revolving per minute. 
- If the gears are rotating at extreme high speed, they will wear out soon -> hence difference of their life (in minutes) will be smaller.
- If the gears are rotating at extreme slow speed, they will last lot longer -> hence difference of their life (in minutes) will be much larger.
Hence we cannot conclude how many days the gears will last long.

(2) The smaller gear revolves 600 times per minute.
 INSUFFICIENT : This information is clearly insufficient. This tells the revolution rate only for smaller gear. No info about larger gear (speed or relative diameter ratio)

Combining (1) & (2)
  SUFFICIENT  : As we know smaller gear has 600 RPM and 1/2 the diameter than larger one -> the larger gear rotate at half the speed i.e. 300 RPM and hence we can find out how many days they can last long.

We can stop at this point (no need to calculate further during actual exam)  but just for sake of curiosity lets calculate further:
No of days larger gear lasts longer than smaller gear =  \frac{6,000,000,000}{24*60*60 minutes} * (\frac{1}{300}-\frac{1}{600}) = 115.74 days.

Hence choice(C) is the answer.

DonQuixote · Answer

Eva above is pretty much spot-on. Note that the larger gear having a diameter twice as long as the smaller gear's means that the larger will turn once for every 4 times the smaller does, but the logical process remains sound. Do also note that as Eva mentioned, you should not bother working out the "complete" answer to the question; once you figure out that you need both the ratio of circumferences (or diameters/radii) and the turn speed of one gear, you can answer the question.

EvaJager · Answer

Why 4 times? Circumference of the circle is  \pi*{Diameter} . 
So, if one of the diameters is twice the other one, circumference is also twice, which means twice more rotations for the smaller gear. No?

metallicafan · Answer

+1 C

Beautiful question. When the small gear rotates one time, the big gear rotates 1/2 its diameter. Therefore, the big gear rotates 300 times in 1 minute.
With that information, we can calculate the time for both gears when they make 6 x 10^9 times.

imhimanshu · Answer

Though I also found the answer to be C. However, I want to ask one question.

The question states that each gear will last for atleast 6,000,000,000 revolutions.

What if the bigger gear last for 9,000,000,000 and smaller gear will last for 6,000,000,000 revolution. In this scenario, we may not be having definite scenario. Isn't it. So, the answer must be E

EvaJager · Answer

the larger gear is guaranteed to last how many days longer than the smaller gear.

The question is about the  guaranteed  number of days and not about possible scenarios.

imhimanshu · Answer

Thanks. Makes Sense now.. :-)

the larger gear is guaranteed to last how many days longer than the smaller gear. The question is about the guaranteed number of days and not about possible scenarios.

greatps24 · Answer

(1) The diameter of the larger gear is twice the diameter of smaller gear.
 This means for Bigger gear # of revolution = 6,000,000,000 / pi * 2d
Smaller gear # of revolution = 6,000,000,000 / pi * d. But we don't know d (diameter)

(2) The smaller gear revolves 600 times per minute.

this gives rate of smaller gear. Not sufficient. We can find

(6,000,000,000 / pi * d) = 600. We can find d.

1 + 2:

Now, we know diameter of Bigger gear
we can find Bigger gear # of revolution per min - Smaller gear # of revolution

torreadortorment · Answer

What if the larger gear revolved faster than the smaller one ?

anupamisrich · Answer

Good point. Could anyone please share thoughts on this? How can the velocities (speed of rotation) be assumed equal for each gear?

chetan2u · Answer

Hi ,
it is given that the two gears are interlocked, so both have to move with same speed..

Hope its clear

LogicGuru1 · Answer

Given in the stimulus , One gear is bigger than the other. 
Both gear will work till 6000000 revolution 
Find the life of the larger gear. 
Life of gear is a function of revolution they makes. "{They will last till 600000000 revolution-given in the question}
Revolution is dependant on the circumference and thus depends on the diameter of the gears. 
Revolution is also dependant on the rate of revolution of gear per second or per minute or per hour

AIM:-Look out for Radius+Rate info

(1) The diameter of the larger gear is twice the diameter of smaller gear.
We got the relative radius info. We don't have exact radius values. And we are still missing rate of revolution
INSUFFICIENT

(2) The smaller gear revolves 600 times per minute.
Nice:- we got the rate info for smaller one since the smaller one and bigger one are connected so their rate will be same; but this statement doesn't talk about diameter
INSUFFICENT

COMBINE
we have the radius as well as rate info for both gear

SUFFICIENT

ANSWER IS C

Ferrarif1286 · Answer

Just looking at the question, (2) is clearly insufficient because you don't know how big the large gear is nor it's revolution rate. (1) you can see the relation of the gears' sizes. However, you can't translate these sizes into time. There's no way to calculate the numbers of days needed for either gear to rotate 6000000000 revs so insufficient. With (1+2) you have the relationship between gear sizes and the time for one gear. Therefore, without doing any calculation, it should be C.

keats · Answer

I am getting the below expression. Can anyone confirm. Thanks!

6 X 10^9/24*60

chetan2u  Can you please confirm.

vishwash · Answer

just one minor point if it can be clarified... in the question it says that ''revolutions'' that the gear will last say n number of revolutions.

but what is required to find out, how may days more larger gear will be.......... i do understand the dynamics of circumference and other things... but how can you assume that in any given day, the gear will keep on rotating 24x365

i mean on some day, there has be to be a rest day also for the machines... request if some one can explain this.... thanks in advance

GMATinsight · Answer

Answer: Option C Please check the attachment. Screenshot 2020-08-05 at 7.43.14 AM.png

Basshead · Answer

As the 2 gears are interlocked, they move at the same speed. I think it's safe to say, unless otherwise mentioned, that the two gears are continuously moving.

Basshead · Answer

Question: In a particular machine, there are 2 gears that interlock; One gear is larger in circumference than the other. The manufacturer of the gears guarantees that each gear will last for atleast 6,000,000,000 revolutions. Assuming that there is no slippage between the 2 gears and that when one gear rotates the other gear also rotates, the larger gear is guaranteed to last how many days longer than the smaller gear.

To answer this question, we need to determine how many revolutions each gear makes in a given time. Lets take a look at the statements:

(1) The diameter of the larger gear is twice the diameter of smaller gear.

larger gear diameter: 2d
smaller gear diameter: d

We're not given the number of revolutions/speed. Therefore, this statement is insufficient.

(2) The smaller gear revolves 600 times per minute.

This statement tells us the speed of the smaller gear; however, we do not have any idea of the speed of the larger gear. Insufficient.

(1&2) We do smaller gear revolves 600 times per minute. We also know that the larger gear has a diameter twice as big as the smaller gear. That means if the smaller gear revolves 600 times per minute, the larger gear revolves 300 times per minute. With the two statements combined, we are able to determine how many days the longer larger gear is guaranteed to last than the smaller gear.

In a particular machine, there are 2 gears that interlock; One gear is

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GMAT Data Sufficiency (DS) Questions

In a particular machine, there are 2 gears that interlock; One gear is

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