Multiple choice

A cistern can be filled by two pipes in 30 and 40 min respectively. Both the pipes are opened at once, when the first pipe must be turned off so that the tank may be just filled in 18 min ?

  1. 12.5 min

  2. 13.5 min

  3. 14.5 min

  4. 16.5 min

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

Let the first pipe be open for x minutes. Pipe 1 rate = 1/30, Pipe 2 rate = 1/40. Pipe 2 is open for 18 minutes. Equation: x/30 + 18/40 = 1. x/30 = 1 - 18/40 = 22/40 = 11/20. x = 30 * 11/20 = 33/2 = 16.5 minutes.