Question 112547
Let x=number of additional gallons needed to fill the tank

The 15 gal alone that were added filled the tank to (3/4-1/8) or 5/8 capacity.  Since the tank was already 1/8 full, only 1/4 now remains to be filled.  So we have the following ratio:
15 is to 5/8 as x is to 1/4 or 2/8
15/(5/8)=x/(2/8)  cross multiply

15(2/8)=(5/8)x  multiply both sides by 8

30=5x divide both sides by 5

x=6 gal





Hope this helps---ptaylor