Ratio story problem?

Question

A water tank can be filled in 7 minutes and emptied in 9 minutes. If the drain is left open when the tank is being filled, how long does it take to fill the tank?

Answer 1

Each minute, 1/7 of the tank is filled and 1/9 drained.

 

1/7 - 1/9 = 1/x

 

Once we find the common denominator, 63x, we don't need it anymore.

 

9x - 7x = 63

2x = 63

x = 31.5

 

It will take 31 minutes and 30 seconds to fill the tank.