`13)`
`2/3:(0,1x)=1 1/3:0,8`
`\to 2/3:(0,1x)=5/3`
`\to 0,1x=2/3:5/3`
`\to 0,1x=2/5`
`\to x=2/5:0,1`
`\to x=4`
Vậy `x=4`
`14)`
`x:0,16=9:x`
`\to x/(0,16)=9/x`
`\to x.x=9.0,16`
`\to x^2=36/25`
`\to x^2=(6/5)^2`
`\to` \(\left[ \begin{array}{l}x=\dfrac{6}{5}\\x=-\dfrac{6}{5}\end{array} \right.\)
Vậy `x\in{-6/5;6/5}`