a) x+30+x=180
2x+30=180
2x=180-30
2x=150
x=75
x+30=75+30=105
b)x-42+x=180
2x-42=180
2x=222
x=111
x-42=111-42=69
c)x+3/5·x=180
5/5·x+3/5·x=180
8/5·x=180
x=180/1:8/5
x=180/1·5/8
x=900/8
x=112°30′
3/5·x=3/5·112,3/1=337,5/5=67°30′
d)90+x+24+x=180
2x+114=180
2x=66
x=33
x+24=33+24=57
e)x+x-3+90=180
2x+87=180
2x=180-87
2x=93
x=46°30′
x-3=46,3-3=43°30′