((12-x)/(1+x))=0.3

$$\frac{12-x}{1+x}=0.3$$

 

Multiply both sides by (1 + x)

$$12-x = 0.3(1+x)$$

 

Multpliy each term in brackets on the right-hand side by the 0.3

$$12-x=0.3+0.3x$$

 

Subtract 0.3 from both sides and add x to both sides

$$12-0.3=x+0.3x$$

 

Collect like terms

$$11.7=1.3x$$

 

Divide both sides by 1.3

$$x=\frac{11.7}{1.3}$$

 

or $$x=9$$

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