method of elimination~20points

1.
10x+12y+1=0 => 20x+24y+2=0 ---(1)
-4x+9y-5=0 => -20x+45y-25=0 ---(2)
(1)+(2):
69y-23=0
y=1/3 ---(3)
Sub (3) into (1):
10x+12(1/3)+1=0
10x=-5
x=-1/2

2.
5s-1=7r => 7r-5s+1=0 ---(1)
{9(r+3)=110-2s => 9r+2s-83=0 ---(2)
(1)x2 +(2)x5:
2(7r-5s+1)+ 5(9r+2s-83)=0
59r-413=0
r=7 ---(3)
Sub (3) into (1):
7(7)-5s+1=0
5s=50
s=10

3.
a/2+b/3=13 => 3a+2b=78 ---(1)
a/3-b/4=3 => 4a-3b=36 ---(2)
(1)x3 +(2)x2:
3(3a+2b-78)+2(4a-3b-36)=0
17a-306=0
a=18 ---(3)
Sub (3) into (1):
3(18)+2b=78
2b=24
b=12