So we know that the decimal place will move one right. Well basically let's assume she could have bought these expressions:
x 10 x 3.50 = 35.00 x 9 x 3.50 = 31.50 + 3.75 = 35.25 x 8 x 3.5 = 28 + 3.75 x 2 = 35.5 x 7 x 3.5 = 24.5 + 3.75 x 35.75
We know that there is a pattern. See how the cents goes .25 up each time. So we know she'd buy 6 reams of white because (6 x 3.5 = 21) + (4 x 3.75 = 15) = 36.
Rewrite the first equation for w then plug it into the second equation. w = 10 - b 3.5(10 - b) + 3.75b = 36.00 35 - 3.5b + 3.75b = 36.00 35 + .25b = 36 Subtract 35 from both sides. .25b = 1 Divide both sides by .25 b = 4 w + b = 10 w + 4 = 10 Subtract 4 from both sides w = 6 White = 6 Blue = 4