Question

A cistern is provided with two pipes a and

b. A can fill a cistern in 20 minutes and b can empty it in 24 minutes. If a and b pipes are opened on alternate minutes, find in how many minutes, the cistern shall be full

Answer 1

a 20 minutes 120l 6l fill in a minute
b 24 minutes 120l. 5l empty in a minute
a-b. 1l in 2 minute...
so to fill the tank fully it requires 240 minutes...