Problem: 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 ?
Rationale: "gear l - - 10 rotations per 60 seconds - - 1 rotation per 6 seconds . gear r - - 40 rotations per 60 seconds - - 4 rotations per 6 seconds . first 6 seconds - - gear l makes 1 rotation . - - gear r 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 9 rotations = > 3 ( 6 seconds later ) = = > 18 seconds . answer : d ) ."
annotated_formula: divide(divide(9, subtract(divide(40, const_60), divide(10, const_60))), const_3)