3x+2.5x=55
（3+2.5）x=55
5.5x=55
x=10
（x-5.5）*8=62.8
x-5.5=62.8/8
x-5.5=78.5
x=78.5+5.5
x=84
6x-2.5x=42
（6-2.5）x=42
x=42/3.5
x=12