If a tap takes 3 minutes to fill up a tub and a hose takes 5 minutes to drain up the same tub, how long will it take to fill up the tub if the hose is draining at the same time?

So, you forgot to post your work and what you thought the answer was. We don't do your assignments for you... but we will help after you show us the work you did do.

This is a simple problem. You have inflow at 1 volume per 3 minutes and outflow at 1volume per 5 minutes. What you need to do is find the lowest common denominator between the two subract the inflow from the outflow and then reduce to a 1 volume per X minutes.

Then sit and ponder why someone would do such a thing, maybe one of sisyphus's kids.