3（x－1.8）＝15.3
x-1.8=5.1
x=6.9
（2.5＋x）÷2＝4.3
2.5+x=8.6
x=6.1
4（x＋3.1）＝16.8
x+3.1=4.2
x=1.1这是正确的吗绝对正确