Aleaky valve on the water meter overcharges the residents for one gallon of water

The overcharge amount w, in this case, would be 1 gallon of water. The time t, in this case, would be 2 1/5 month. Then the equation to count the overcharge amount would be: 
w *1 = t *2 1/5 
w = 2 1/5 t 

Then, for 7 gallons of water, you need to replace w with 7. The calculation would be:
w = 2 1/5 t 
7= 2 1/5 t
t= 7/ (2 1/5)= 3.18 months

... what’s the expression