Worker a finishes the job in 10 hours; worker b finishes it in 12 hours, and worker c finishes it in 15 hours.

1. after a worked alone for two hours, and then b worked alone for 4 hours, how many hours will it take c alone to finish the remaining work?

it takes 40 minutes to fill a water tank, and 60 minutes to drain it.

2. if we try to fill the tank when empty while draining it for 1 minute, what fraction of the tank will be filled?

if they ask me that the tank is 2/3 full how will i find the minutes it will take to drain? do i divided 60?