The problems is related to simple applied mechanics.

There are two wheels attached to each other with belt.

Smaller one: Engine
Bigger one: Mixer

Engine wheel:-
RPM: 60 (rotations per minute)
radius: 2 inches.
length of belt covered in one minuter = 2.pi.r.rpm = 2 x pi x 2 x 60

Mixer wheel:-
RPM: 8
radius: R (unknown)
length of belt covered in one minuter = 2.pi.R.rpm = 2 x pi x R x 8

Since both wheels are attached with single belt therefore length of belt covered by both wheels should be equal.

=> 2 x pi x 2 x 60 = 2 x pi x R x 8
=> R = 15

Therefore, diameter of bigger wheel (mixer) should be = 30 inches.

Hope this helps...

thanks codesnooker