(12/25 - 4/15) + 15/16 x 4/15

Решение:
12/25 - 4/15 = (123)/(253) - (45)/(155)
= 36/75 - 20/75
= 16/75

15/16 x 4/15 = (151)/(161) x (41)/(151)
= 15/16 x 4/15
= (15x4)/(16x15)
= 60/240
= 1/4

Итак:
(12/25 - 4/15) + 15/16 x 4/15 = 16/75 + 1/4
= (16x4 + 75) / 75x4
= 144/300
= 6/15
= 2/5

Ответ: 2/5

5/7 x (21/20 - 7/30) + 16/21 : 8/7

Решение:
21/20 - 7/30 = (213)/(203) - (72)/(302)
= 63/60 - 14/60
= 49/60

5/7 x 49/60 = (5x49)/(7x60)
= 245/420
= 49/84

16/21 : 8/7 = 16/21 x 7/8
= (16x7)/(21x8)
= 112/168
= 2/3

Итак:
5/7 x (21/20 - 7/30) + 16/21 : 8/7 = 49/84 + 2/3
= (49x3 + 84x2) / 84x3
= 147 + 168 / 252
= 315/252
= 5/4

Ответ: 5/4