How much of a 90% alloy must be combined with 70% gold alloy in order to get 60 ounces of an 85% gold alloy?

X = amount of 90% alloy

y = amount of 70% alloy

x + y = 60

0.9x + 0.7y = 0.85*60

0.9x + 0.7 (60 - x) = 0.85*60

(0.9 - 0.7) x = (0.85 - 0.7) * 60

x = (0.85 - 0.7) * 60 / (0.9 - 0.7)

x = 45 ounces

y = 60 - 45

y = 15 ounces