Subject: mathqa
Question: circular gears l and r start to rotate at the same time at the same rate . gear l makes 10 complete revolutions per minute and gear r makes 40 revolutions per minute . how many seconds after the gears start to rotate will gear r have made exactly 9 more revolutions than gear l ?
Option: 
A: b ) 8
B: d ) 18
C: a ) 6
D: e ) 15
Answer: B