Question

A pipe can fill a tank in 10 hours,while another pipe can empty the tank in 20 hours.If both the pipes are opened together,the tank will be completely fill in?

Answer 1

1/10 - 1/20,

(2-1)/20,

1/20,

then
answer will be 20 hrs

Answer 2

20 hours time taken by the both pipes to fill the water tank.