Question 901579
Filling or draining a tank is 1 job

Filling 6 minutes
so it does 1/6 of the job in 1 minute

draining -18 minutes
so it does 1/18 of the job in 1 minute

If both are open effective rate will be

1/6 - 1/18

2/18

= 1/9

so 1/9 of the job is done in 1 minute

It will take 9 minutes to fill the tank