i am stuck on the following problem. here is a pic of the problem

http://imgur.com/CvLAT0P

any help would be appreciated

I am going to guess 10 becuase it takes 15 hours to drain with both pipes open but with not the pipe open that fills, its only draining so it wouldnt take 5 hours more to drain. I subtrated 5 from the 15. I couls be wrong,

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