arithmetic - a tank is filled in 5 hours by three pipes a b and c the pipe c is twice as fast as b and b is twice as fast as a how much time will pipe a alone take to fill the tank - skillgun

Suppose pipe A alone takes x hours to fill the tank.
Then, pipes B and C will take x/2 and x/4 hours respectively to fill the tank.
1/x + 2/x + 4/x = 1/5
7/x = 1/5
x = 35 hrs.