200-x=150

200-x+x=150+x

150+x=200

X+150-150=200-150

X=50

(39)(x-140)÷70=4

(x-140)÷70×70=4×70

X-140=280

X-140+140=280+140

X=420

（40）20-9x=2

9x=20-2

9x=18

9x÷9=18÷9

X=2

(41)x+19.8=25.8

X+19.8-19.8=25.8-19.8

X=6

(42）5.6x=33.6

5.6x÷5.6=33.6÷5.6

X=6

（43）9.8-x=3.8

9.8-x+x=3.8+x

3.8+x=9.8

X=9.8-3.8

X=6

（44）75.6÷x=12.6

X=75.6÷12.6

X=6

（45）5x+12.5=32.3

5x+12.5-12.5=32.3-12.5

5x=19.8

5x÷5=19.8÷5

X=3.96

（46）5(x+8)=102

5x+40=102

5x+40-40=102-40

5x=62

5x÷5=62÷5

X=12.4