Hello! I know somebody might have already answered this question, but I figured I could add more to it. In order to help you know whenever a question shows up like this again, I'll make an ordered situation for you.
Let's go ahead and make the situation!
The Situation: Let's call the person trying to figure this problem out by Alison. Alison turns on the water and allows it to run in the tub for about 9 minutes. She then turns the water off while she runs to answer the front door. She soon comes back and allows water to run into the tub for a couple more minutes before she takes a short bath. After the bath, Alison allows the water to completely drain from the tub. That's how the problem would've went, but your question.
When you're asking the question you're also trying to figure out how it would've gone. So, since you tried following the paragraph I made for the picture/problem I can show how'd it help you comprehend it better. After Alison turns on the water, turns it back off then comes back to run the water AGAIN.. She finally drains it from the tub, which is what the picture exactly shows. BUT when did she turn it off?!!!
Look at the problem, at the point where the Water level(in) starts decreasing. We know that we end with the Water level(in) at 0, but how'd it get there? The point at 12 in time is when it starts decreasing! But, from there it continues until it's at 15(min)
THE FINAL ANSWER: At 12(min) it decreases until it hits 15(min) which is when the tub is empty. So, if you do common math 15(min) - 12(min) is 3(min) So, it took 3(min) to empty the tub when it was the most full.
