A community water tank can be filled in 18hr by the town office well alone and in 22hr by the high school will alone. How long will it take to fill the tank if both wells are working?

Time taken by town office well to fill the tank = 18 hrs.

So quantity filled in 1 hr = 1/18 th part of the tank 

Time taken by high school well to fill the tank = 22 hrs.

So quantity filled in 1 hr = 1/22 nd part of the tank 

When both the wells are open, quantity filled up in 1 hr =1/18 + 1/22

= (11+9)/198 [taking LCD]

= 20/198 part of the tank 

So time needed by both the wells to fill the tank = 198/20 hrs

= 9.9 hrs 

= 9 hrs 54 min