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Ответ:

4)-Б 12/21=4/7

5)

а)348-y=42

-y=42-348

-y=-306

y=306

б)x+7,19=15,4

x=15,4-7,19

x=8,21

в)7 3/4x=3 1/6

7 3/4=7*4+3=31/4

3 1/6=3*6+1=19/6

31/4x=19/6|*4/31

x=38/93

г)12/17y=1|*17/12

y=17/12

6)(10 2/5-2 2/3:1 1/7)+3,2:0,04

(10 2/5-2 2/3*1/7)+3,2:0,04

10 2/5=10*5+2=52/5

2 2/3=2*3+2=8/3

(52/5-8/3*1/7)+80

(52/5-8/21)+80

1052/105+80

9452/105