(15/98 - 17/28) : 4 5/21 + (4/40 - 3/20) * 21 3/7 = - 1 5/28
1) 15/98 - 17/28 = 30/196 - 119/196 = - 89/196
2) 4/40 - 3/20 = 2/20 - 3/20 = - 1/20
3) - 89/196 : 4 5/21 = - 89/196 : 89/21 = - 89/196 * 21/89 = - 21/196 = - 3/28
4) - 1/20 * 21 3/7 = - 1/20 * 150/7 = - 150/140 = - 1 1/14
5) - 3/28 + (- 1 1/14) = - 3/28 - 1 2/28 = - 1 5/28