math
posted by vincent on .
One wheel has a diameter of 30 inches and a second wheel has a diameter of 20 inches. The first wheel traveled a certain distance in 240 revolutions. In how many revolutions did the second wheel travel the same distance?
not just an answer please
A.120
B.160
C.360
D.420
E.480

short way:
the number of revolutions varies inversely with the diameter of the wheel
i.e. the larger the wheel the smaller the number of revs.
R1/R2 = C2/C1
240/R2 = 20/30
20R2 = 7200
R2 = 360
or
distance covered by first wheel = 2π(15)(240) = 7200π inches
circumference of 2nd wheel = 2π(10) = 20π inches
for the same distance of 7200π inches
number of revolutions = 7200π/(20π) = 360 revs 
c.360