2/3：x=1/2:3/8
2/3：x=1/2×8/3
2/3：x=4/3
x=2/3÷4/3
x=2/3×3/4
x=1/2
3/4x-(1-80%)x=1.05
3/4x-(1-4/5)x=21/20
3/4x-1/5x=21/20
15/20x-4/20x=21/20
11/20x=21/20
x=21/11