A+B+C = 100%, which is a good start!

Product A: 10% of $500 = $50, $50/$1.20 = 41.666... (ie 41 2/3)

Product B: 25% of $500 = $125, $125/$5 = 25

Product C: 65% of $500 = $325, $325/$8 = 40.625

Now, I imagine you can't buy a fraction of a Product, so a decision has to be made.

And by the sound of it, you are **required** to have the 10%/25%/65% breakdown, but you are **allowed** to spend less than $500, am I right? (if it was me, I would just buy 25 of B, 40 of C, and spend the rest on A, but that would not give the exact percentages)

So, if you want to play in Excel, then I would lay the formulas out like I did up top, and then reduce the $500 amount until it all works out.