Let after n week both tanks have same quantity of water .
for first tank ============= tank hold 540 gallon of water , leaking of water 4 gallon /week so, in n week water leak from the tank =4× n = 4n
now, after n week quantity of water in the tank = 540 -4n
for second tank ============== quantity of water in the tank =720 gallon leaking of water at the rate = 7gallon/week so, after n week water leak from the tank = 7× n =7n
after n week quantity of water rest in the tank = 720 -7n
now , according to question ,
water in first tank = water in second tank 540 -4n = 720 -7n
7n -4n = 720 -540
3n = 180
n = 60
hence , after 60 week both tank have same quantity of water.