GMAT Questions Chat Group

I think, it is option C only.
Take choice A, it states that wheel B rotates 3times faster than wheel A.
With choice A alone, we cannot come to the conclusion in how many seconds wheel B completes 6 times more rotations than wheel A.
So eliminate A & D options.

Take choice B, it states that average revolution of both the wheels are 40 / minute.

So , average revolution of wheel (A + B)/2 = 40
So max possible is A+B = 80 revolutions / minute.
This is also not helpful

But if we substitute choice A , i.e Wheel B has rate of rotation of 3 times than wheel B
Wheel A’s rate of rotation = B/3

Sub, that rate in Choice B ,

A+B = 80
(B/3)+B=80
B+3B=240
B=60 revolutions per minute
Therefore, Wheel A’s rotation is 20 rotations per minute.

We can therefore deduce that after 9 seconds, Wheel A would have rotated 3 times whereas 9 revolution by wheel B, therefore there’s a difference of 6 revolutions between both the wheel.

So option C