Question 977607
You should use the density of sugar solution in water  (sucrose solution):
See the data here:  http://homepages.gac.edu/~cellab/chpts/chpt3/table3-2.html


That gives 15% sucrose is 1.0592 grams/cc.


Your calculation starts with 1 GALLON of 15% sucrose.


Another page online gives a density conversion calculator,
http://www.unit-conversion.info/density.html


That will allow you to take the factor 8.345 to change grams/cc to pounds per gallon.  This means,  {{{1.0592*8.345=8.839*poundsPerGallon}}} for 15% sucrose.


USING THE CALCULATED SOLUTION INFORMATION


You want to have 1 gallon of 15% sucrose solution.
1 gallon of 15% sugar is 8.839 POUNDS of solution.


How much of this is sugar?  
{{{(0.15)(8.839)=1.3259*poundsSugar}}}


How much of this is water?
{{{8.839-1.3259=7.513*poundsWater}}}


Read very carefully; these are the quantities of water and sugar IN POUNDS to use.  How you actually measure the ingredients depends on your desired choices.  You may want to convert the sugar quantity into grams, or into pounds & ounces.   You may want to convert your water quantity into ounces, or into FLUID OUNCES  (which should never be confused with "ounces").