x-30-(x-30)*0.7=0.

x-30-(x-30)*0.7=0.
x-30-(x-30)*0.7=0.

x-30-(x-30)*0.7=0.
x-30-(x-30)*0.7=0.1x
x-30-0.7x+21=0.1x 去括号,注意前面是负号,要变号
x-0.7x－0.1x＝30-21 移项,也要注意变号
0.2x＝9 合并同类项
x＝45 系数化为1,两边除以0.2

45

x-30-(x-30)*0.7=0.1x
x-30-0.7x+21=0.1x
x-0.7x-0.1x=30-21
0.2x=9
x=9÷0.2
x=45

X=45
原式可化为（x-30）*0.3=0.1X
所以 0.3X-9=0.1X
所以 0.2X=9
X=45