A large pipe can fill a tank in 10 minutes, and a smaller pipe can fill the tank in 20 minutes. How long will it take to fill the tank if the both pipes are used at the same time?

In one minute large pipe can fill 1/10 th  of tank.

In one minute small pipe can fill 1/20 th of tank.

In one minute if both pipes are opened they can fill (1/10+1/20) th of tank.

=(1+2)/20 =3/20 th of tank.

for It takes 20/3 days to fill the tank.

20/3 = 6 2/3 days.