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