@Kate-,
Leopard, to get the C value in a / (b - c) = d, substitute in the B values you already have (48, 120) and the D values you already have (90, 30), then make the two equations equal (you have to multiply the second one by 3 because 90 / 30 = 3). Then cross multiply the a / (b - c) part of each equation, then you'll get what C is eqaul to.
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I never really understood this type of math in Math Class