Word problem

Question

An oil tanker can be emptied by the main pump in 4 hours. An auxiliary pump can empty the tanker in 6 hours. If the main pump is started at 6 PM?, when should the auxiliary pump be started so that the tanker is emptied by 9 PM??

Answer 1

The main pump works for 3 hours (9 - 6).

In that time it emptys 3/4 of the tank.

The auxiliary takes 6 hours to empty the whole tank.

How much time does it need to empty 1/4 of the tank?

 

(1/4)(6) - 1.5

 

The auxiliary must be opened at 7:30 (1.5 hours before 9)