3.5*（3+X）=12 （5-x）*4.2=20 解方程3.5*（3+X）=12 （5-x）*4.2=20 解方程

3.5*（3+X）=12
3.5 * 3 + 3.5x = 12
10.5 + 3.5x = 12
3.5x = 12 - 10.5
3.5x = 1.5
x = 1.5÷3.5
x=3/7
（5-x）*4.2=20
5 * 4.2 - 4.2x = 20
21 - 4.2x = 20
4.2x = 21 - 20
4.2x = 1
x = 1÷4.2
x = 5/21

3.5*（3+X）=12
10.5+3.5x=12
3.5x=12-10.5
3.5x=1.5
x=3/7
（5-x）*4.2=20
21-4.2x=20
4.2x=21-20
x=1÷4.2
x=5/21

3.5*（3+X）=12
3+x=12÷3.5
3+x=24/7
x=24/7-3
x=3/7
（5-x）*4.2=20
5-x=20÷4.2
5-x=100/21
x=5-100/21
x=5/21