Question

one tap fills a tank in 20 minutes and another tap fills in 12 minutes the tank being empty and if both taps are opened together in how many minutes the tank will be full

Answer 1

1st tap fills it in 20 mins.therefore it does 1/20 amount of work in a minute.
2nd tap fills it in 12 mins.therefore it does 1/12 amount of work in a minute.
if both taps are opened together then amount of work in a minute=1/20+1/12=2/15
therefore if the taps do 2/15 work in a minute,time taken to fill the tank is 15/2=7.5