Question

A new type of pump can drain a certain pool in 6 hours. An older pump can drain the pool in 14 hours. How long will it take both pumps working together to drain the pool?

Answer 1

Needs to use reciprocals:

time*(1/6+1/14) = 1 pool,

time = 1/(1/6+1/14) = 84/20 = 21/5 = 4.2 hours