Решит уравнение : 4-(4 21/40-5,25): 1 9/40

4 - (4 21/40 - 5,25) : 1 9/40 = 4 29/49

1) 4 21/40 - 5 1/4 = 4 21/40 - 5 10/40 = - 29/40
2) - 29/40 : 1 9/40 = - 29/40 : 49/40 = - 29/40 * 40/49 = - 29/49
3) 4 - (-29/49) = 4 + 29/49 = 4 целых 29/49

Пояснения:
5,25 = 5 целых 25/100 = 5 целых 1/4  (25/100 сократили на 25)
1/4 = 10/40 - доп.множ. 10
4 21/40 - 5 10/40 = = - (5 10/40 - 4 21/40) = - (4 50/40 - 4 21/40) = - 29/40
1 целая 9/40 = 40/40 + 9/40 = 49/40