`a)` `x/15=(-2)/(3,5)`
`to 3,5x=(-2).15`
`to 3,5x=-30`
`to x=(-30)/(3,5)`
`to x=-60/7`
Vậy `x=-60/7`
`b)` `16/x=x/25`
`to x^2=16.25`
`to x^2=400`
`to x=+-20`
Vậy `x=+-20`
`c)` `(0,5)/(0,7)=(-0,1)/(5x)`
`to 0,5.5x=(-0,1).0,7`
`to 2,5x=-0,07`
`to x=(-0,07)/(2,5)=-7/250`
Vậy `x=-7/250`
`d)` `(1,2)/(x+3)=5/4`
`to 5(x+3)=1,2.4`
`to 5x+15=4,8`
`to 5x=4,8-15`
`to 5x=-10,2`
`to x=(-10,2):5`
`to x=-2,04`
Vậy `x=-2,04`