a + b = 20 b + c = 31 a + c = 27 2a + 2b + 2c = 20 + 31 + 27 2 ( a + b + c ) = 78 a + b + c = 78 : 2 a + b + c = 39 20 + c = 39 c = 39 - 20 c = 19 a + c = 27 a = 27 - 19 a = 8 b + c = 31 b + 19 = 31 b = 31 - 19 b = 12 verificam 8 + 12 + 19 = 39 a x b x c = 8 x 12 x 19 = 96 x 19 = 1824