1/4x=13/20
x=13/20*4=13/5
1/4+x=13/20
x=13/20-5/20=7/20
x÷1/4=13/20
x=13/20*1/4=13/80
2/3x-1/5x=21
(10/15x-3/15x)=21
7/15x=21
x=21*15/7=45
3/4x=5
x=5*4/3=20/3
x-7/9x=400
2/9x=400
x=400*9/2=1800
1.8+1.2x=24
1.2x=22.2
x=18.5
0.5x+2/5x=0.72
0.9x=0.72
x=0.8
3/4x-2/5x=7/8
15/20x-8/20x=7/8
7/20x=7/8
x=5/2