Question 27746
One approach to this problem is first find the hourly rate of fill for each pipe. Let's call the first pipe A and the second pipe B.

If pipe A can fill the tank in 7.5 (15/2) hours, then it can fill 2/15 of the tank in 1 hour.
If pipe B can fill the tank in 10 hours, then it can fill 1/10 of the tank in 1 hour.

If pipe B were to be left open for 8 hours, then it would fill 8 X (1/10) of the tank. 
So, after 8 hours, 4/5 of the tank would be filled leaving 1/5 of the tank to be filled by pipe A.

Now, pipe A can fill 2/15 of the tank in 1 hour, so how long would it take to fill the 1/5 of the tank?

Divide 1/5 by 2/15 {{{(1/5)*(15/2) = 3/2}}}hours.

Pipe A would need 1.5 hours to finish filling the tank.