x/8+x÷4/3=21
x/8+3/4x=21
7/8x=21
x=24
x÷3又1/3-1/5x=20
3/10x-1/5x=20
x/10=20
x=200
2/3x+x÷2/3=4/5
=2/3x+3/2x=4/5
13/6x=4/5
x=24/65