please help me answer this anyone.. haha

## one pipe can fill a tank in 5 hours. another can empty the tank in 8 hours. If both pipes are left open, how long will it take to fill the tank??

Tutors, please sign in to answer this question.

# 1 Answer

Let V =the volume of the tank

The first pipe fills 1/5 of the tank per hour: Rate 1 = +V/5

The second pipe empties 1/8 of the tank per hour: Rate 2 = -V/8

Rate 1 is positive because it's filling the tank. Rate 2 is negative because it's emptying the tank. We leave each pipe open for a time t, and we want the tank to be full (volume = V) at the end of that time.

(Rate 1)*time + (Rate 2)*time = full tank (V)

(V/5)t + (-V/8)t = V

(1/5 - 1/8)Vt = V (Factor out V and t)

(3/40)t = 1 (Subtract the fractions, divide both sides by V)

t = 40/3 = 13 1/3 hours (13 hours 20 minutes)

Check:

(V/5)(40/3) + (-V/8)(40/3) = (8/3)V - (5/3)V = (3/3)V = V It fills the tank!

## Comments

Comment