20x+20-15y-30=24x-24y /*60
3x-9-4y+12=24y-12x /*12

-4x+9y-10=0/-24x+24y
15x-28y-9=0/+12x-24y

-4x+9y=10 /+10
15x-28y=9 /+9

-4x+9y=10
-4x+(7 and 7/15)y=-2.4 /:-3.75 (got -3.75 by doing 15/-4)

now, i add the two together:

(16 and 7/15)y=7.6

and finally:

y=6/13

and it's supposed to be 6