x-2.875=1/8
x=0.125+2.875
x=3
70％x=28
0.7x=280
x=400
3.8+4x=23
4x=23-3.8
4x=19.2
x=4.8
x/2.5=0.4:9
9x=2.5×0.4
9x=1
x=1/9