Ametallurgist has an alloy with 5
%
titanium and an alloy with 30
%
titanium. he needs 100 grams of an alloy with 15
%
titanium. how much of each alloy should be mixed to attain the 100 grams of alloy with 15
%
titanium?

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