a) 5y+(8y+9)=100

б) x-(50-x)=12

в) 30-x=3(20-x)

а) 13 y+9 = 100

13 y-91 = 0

y = 7

б) 2 x-50 = 12

2 x-62 = 0

x = 31

в)

30-x = 60-3 x

2 x-30 = 0

x = 15

5y+(8y+9)=100

5y+8y+9=100

13y+9=100

13y=100-9

13y=91

y=91/13

y=7

x-(50-x)=12

x-50+x=12

2x-50=12

2x=50+12

2x=62

x=62/2

x=31

30-x=3(20-x)

30-x=60-3x

2x-30=0

2x=30

x=30/2

x=15