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 15 more revolutions than gear P? Choose the most appropriate option:
A) 12  B) 15  C) 20  D) 24  E) 30

Rationale: gear q makes 30 more revolutions each 60 seconds . the time to make 15 = 30 / 2 more revolutions is 60 / 2 = 30 seconds . the answer is e .
Answer: E