Let c = grams of copper to be added
(grams of silver) / (grams of copper and silver) < = .90
387+%2F+%28c+%2B+387%29+%3C=+.9
387+%3C=+.9%28c+%2B+387%29
387+%3C=+.9c+%2B+348.3
.9c+%3E=+387+-+348.3
.9c+%3E=+38.7
c+%3E=+43g
At least 43 grams of copper must be added
check:
387+%2F+%28c+%2B+387%29+%3C=+.9
387+%2F+%2843+%2B+387%29+%3C=+.9
387+%2F+430+%3C=+.9
.9+%3C=+.9
