(11,2-3 5/12+4,3-7 19/30):7 7/12-5 5/6)

осталось 15 минут

(11,25 - 3 5/12 + 4,3 - 7 19/30) : (7 7/12 - 5 5/6) = 2 целых 4/7

1) 11,25 - 3 5/12 = 11 1/4 - 3 5/12 = 11 3/12 - 3 5/12 = 10 15/12 - 3 5/12 = 7 10/12 = 7 5/6

2) 7 5/6 + 4,3 = 7 5/6 + 4 3/10 = 7 25/30 + 4 9/30 = 11 34/30  

3) 11 34/30 - 7 19/30 = 4 15/30 = 4 1/2

4) 7 7/12 - 5 5/6 = 6 19/12 - 5 10/12 = 1 9/12 = 1 3/4

5) 4 1/2 : 1 3/4 = 9/2 : 7/4 = 9/2 * 4/7 = (9*2)/(1*7) = 18/7 = 2 4/7

Пояснения:

0,25 = 25/100 = 1/4 - сократили на 25

1/4 = 3/12 - доп.множ. 3

10/12 = 5/6 - сократили на 2

5/6 = 25/30 - доп.множ. 5

0,3 = 3/10

3/10 = 9/30 - доп.множ. 3

15/30 = 1/2 - сократили на 15

9/12 = 3/4 - сократили на 3

4 целых 1/2 = (4*2+1)/2 = 9/2

1 целая 3/4 = (1*4+3)/4 = 7/4