two pipes filling a pool. how many hours does it take?

Question

A pool can be filled by one pipe in 2 hours and by a second pipe in 4 hours.  How long will it take to fill the pool using both pipes?

Answer 1

Work=rate times time

 

work = 1 (you are filling one pool)

rate =1/2 +1/4 (the rates of the individual pipes)

solve for t

 

1 =3/4(t) 

 

4/3 = t  

4/3 of an hour or 1 hour and 20 minutes