Multiple choice

Two pipes A and B can fill a tank in 6 minutes and 7 minutes, respectively. If they are opened for one minute alternately, beginning with A, in how much time will they fill the tank?

  1. 7 minutes

  2. 65/8 minutes

  3. 61/8 minutes

  4. 63/7 minutes

Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

A fills 1/6 per min, B fills 1/7 per min. In 2 mins, they fill 1/6 + 1/7 = 13/42. In 6 mins (3 cycles), they fill 3 * 13/42 = 39/42. Remaining is 3/42 = 1/14. A fills 1/14 in (1/14) / (1/6) = 6/14 = 3/7 mins. Total time = 6 + 3/7 = 6 3/7 mins.