Pump A can fill a tank in 4 hours and pump B can fill it in 12 hours. How long will they take working together?

Correct answer to question 88 (Arithmetic Reasoning): 3 hours

Method

Add the rates rather than the times. Pump A fills 3/12 of the tank an hour and pump B fills 1/12, giving 4/12 or one third per hour, so the tank fills in 3 hours. Averaging the two times to get 8 is the standard trap.