All I know are:
11. 2+5=7, in reverse, 25 and 52 thus, resulting in 52-25=27..
18. 3x+7(500)=20,000
3x+3,500=20,000
3x=20,000-3,500
3x=16,500
3x/3=16,500/3
x=5,500
7%x500=3,500
3%x5,500=16,500
3,500+16500=20,000...
20. x=6; y=6
x+5y+2x/4=39
6+5(6)+2(6)/4=39...