Ametallugarist has an alloy with 6% titanium and an alloy with 20% titanium. he needs 100 grams of an alloy with 13% titanium. how much of each alloy should be mixed to attain the 100 grams of alloy with 13% titanium?

so.. .whatever those amounts are, of "x" and "y", for the mixture,
they must add up to 100 grams, or x + y = 100

and the titanium percentage of that sum, must yield a 13 grams of titanium on those 100 grams

thus 

do the substitution, and solve for "x",

what's "y" amount?  well, y = 100 - x