) An inlet pipe can fill an empty tank in 5 hours by itself, and an outlet pipe can drain the same tank when full in 7 hours by itself. If the tank is two-thirds full when the valves for both pipes are opened, how many minutes will it take to fill the tank?

Answer 1

To fill the whole tank in x hours,

1/5 - 1/7 = 1/x

but you only have to fill 1/3, so adjust accordingly.

Answer 2

1/5 - 1/7 = 2/35
so to fill full tank, time req = 17.5 hrs
17.5 hrs / 3 = 5 hrs & 50 min, so just to fill 1/3 of the tank, time req is 5 hrs, 50 min

Answer 3

—> 1/3 tank needs to be filled (of 7*5 = 35 units)
—> Both pipes together can do = (7-5) = 2 units/hour
—> ( 35/2*3 = 35/6 hrs = 5hrs 50 minutes = 350 minutes) 🙂