(C) 2008 GMAT Club - [t]m05#23[/t]

[4meonly]

A machine consisting of 3 connected wheels is mixing concrete. The engine is running at the speed of 60 rpm and the mixer is mixing at the rate of 8 rpm. What is the diameter of the mixing wheel if the radius of the smaller (drive) wheel is 2 inches?

12
15
25
30
32

OA D

I can not imagine the question. Can anybody post a picture? Thank you in advance!

[logitech]

Well that three wheel is confusing you. Since they are all connected they cover the same distance:

2xpix2x60rpm=2xpixRx8rpm

2R=D=30

[gaggleofgirls]

You can just ignore the third wheel here as it is not contributing anything and picture it as I did in the attached PDF.

The way I thought of it is that it takes the smaller wheel 60 revolutions to drive the larger wheel 8 revolutions, so the circumference of the smaller wheel is 8/60th of the that of the larger wheel. Therefore the circumference of the larger wheel is 7.5x that of the smaller wheel.

Radius of the smaller wheel is 2, so the Diameter is 4
Circumference of the smaller wheel is 4*Pi
Circumference of the larger wheel is 7.5 * 4 * Pi = 30* Pi
But, we want the Diameter of the larger wheel, so the answer is just 30.

-Carrie

[dnkcdnguy]

I followed the same logic as gaggleofgirls and came up with 30.

Thanks for the explanation gaggleofgirls; it helped me simplify some parts of mine..