Pipes A and B can fill a cistern in 20 and 60 minutes respectively. They are opened an alternate minutes. Find how many minutes, the cistern shall be full?  Choose the correct answer: A) 30 B) 45 C) 32 D) 18 E) 19
:   1/20 + 1/60 = 1/15   15 * 2 = 30.30nswer:30
<result>A</result>
Pipes A and B can fill a cistern in 20 and 60 minutes respectively. They are opened an alternate minutes. Find how many minutes, the cistern shall be full?