x-4/5x=20
1/5x=20
x=20*5
x=100
2/3x+1/2x=49
(2/3+1/2)x=49
(7/6)x=49
x=49÷(7/6)
x=42
75+7/10x=102
7/10x=102-75=27
x=27÷(7/10)
x=270/7
2x+9/2=12
2x=12-9/2=15/2
x=(15/2)÷2
=15/4