so hmm notice, we use the decimal format for the percent, namely 15% is just 15/100 and 5% is just 5/100 and so on
whatever the amounts of "x" and "y" are, they must add up to 100 grams
thus x + y = 100
and whatever the concentrated amounts are, they'll add up to (100)(0.15)
thus
solve for "x", to see how much 5% alloy will be needed
what about "y"? well y = 100 -x