Explanation
To solve the question, we will be aware that
There are 3 hours between 6PM and 9PM ,
The main pump's emptying rate is 1/4 tanker per hour. It will be open
the entire 3 hours, so it will empty 3/4 of the tank.
this means that the auxiliary pump must empty the remaining 1/4
Thus
[tex]rate\text{ in tanker per hour }\times time\text{ required=}\frac{1}{4}tank[/tex]Thus, we will have
[tex]\frac{1}{4}\times3\text{ hours =45 minutes}[/tex]Therefore
The auxiliary pump should start by 6pm + 45 minutes = 6:45pm