Given:
Candy x = 0.90 per pound
Candy y = 0.40 per pound
100 pounds * 0.85 per pound = 85
x + y = 100
0.90x + 0.40y = 85
x = 100 - y
0.90(100 - y) + 0.40y = 85
90 - 0.90y + 0.40y = 85
-0.50y = 85 - 90
y = -5 / -0.50
y = 10
x = 100 - y
x = 100 - 10
x = 90
0.90x + 0.40y = 85
0.90(90) + 0.40(10) = 85
81 + 4 = 85
85 = 85