1.31-2z=15

-2z=15-31

-2z=-16

z=-16/2

z=8

2.7-y=15

-y=15-7

-y=8

y=8

3.3+0.1x=14

0.1x=14-3

0.1x=11

x=11/0.1

x=11*10

x=110

4.1.2t+0.4=1 

1.2t=1-0.4

1.2t=0.6

t=0.6/1.2

t=6/12

t=0.5