Выполните действия:(12 3/7-18 3/14 + 15 3/28):3=?

1) 12 3/7 - 18 3/14 = 12 6/14 - 18 3/14 = 12 6/14 - 17 17/14 = - 5 11/142) - 5 11/14 + 15 3/28 = - 5 22/28 + 15 3/28 = - 5 22/28 + 14 31/28 = 9 9/28

87/7 - 255/14 + 423/28 = 4*87/7 - 2*255/14 + 423/28 = 348 - 510 + 423 = 261/28261/28 / 3 = 261/28 * 1/3 = 87/28 = 3 3/28