Revolutions

[Atonio79]

Circular gears P and Q start rotating at the same time at constant speeds. Gear P makes 10 revolutions per minute, and gear Q makes 40 revolutions per minute. How many seconds after the gears start rotating will gear Q have made exactly 6 more revolutions than gear P?

a. 6
b. 8
c. 10
d. 12
e. 15

[Retake_GMAT]

P = 10/60 or 1R/6sec
Q = 40/60 or 2R/3sec = 4R/6sec

which means Q gains 3Revolutions every 6 seconds. For Q to gain 6, we need another 6seconds to pass. Which means 2sets of 6seconds have passed or 12seconds.
Ans : 12seconds

[Anson800]

Relative speed=30R/S
6 more rev=12 seconds

[GmatG]

Relative speed of gear will be 3R/6
Hence 6 rev will be completed in 12 sec.

[Atonio79]

D is the correct answer...Thanks!