A+B+D=255 B+15=A D+12=B A= A + B + D = 255 B + 15 = A D + 12 = B A = ? Rearrange (3) to solve for D by subtracting 12 from each side: D = B - 12 Substitute (2) and (5) into 1 (B + 15) + B + (B - 12) = 255 Combine like terms: 3B + 3 = 255 Using our equation solver, b = 84 Substitute b = 84 into equation (2): A = 84 + 15 A = 99