Circular gears L and R start to rotate at the same time at the same rate. Gear R makes 10 complete revolutions per minute and gear L makes 40 revolutions per minute. How many seconds after the gears start to rotate will gear L have made exactly 6 more revolutions than gear R?
Pick one.
A) a) 6 B) b) 12 C) c) 10 D) d) 14 E) e) 15
GearR -- 10 rotations per 60 seconds -- 1 rotation per 6 seconds.   Gear L -- 40 rotations per 60 seconds -- 4 rotations per 6 seconds.   First 6 seconds -- Gear R makes 1 rotation. -- Gear L makes 4 rotations -- Net difference -- 3 rotations   Hence every 6 seconds the difference between the number of rotations of R and L gear is 3 units.   Required net difference should be 6 rotations =&gt; 2 (6 seconds later) ==&gt; 12 seconds.   Answer: b) 12
<result>B</result>
Circular gears L and R start to rotate at the same time at the same rate. Gear R makes 10 complete revolutions per minute and gear L makes 40 revolutions per minute. How many seconds after the gears start to rotate will gear L have made exactly 6 more revolutions than gear R?