two numbers with a sum of 16 and a difference of 39 x+y = 16 x - y = 39 use substitution x - y + y = 39 + y x = 39+y (39 + y) + y = 16 39 + 2y = 16 39 - 39 + 2y = 16 - 39 2y = - 23 y = -23/2 substitute back in x - (-23/2) = 39 x = 39 - (23/2) x = (78 - 23)/2 = 55/2 CHECK: 55/2 + -23/2 = 32/2 = 16 55/2 - (-23/2) = 55/2 + 23/2 = (55+23)/2 = 78/2 = 39