`a) y-0.25%= 1/2 `
`⇔y-1/400=1/2`
`⇔y=1/2+1/400`
`⇔y=201/400`
Vậy `y=201/400`
`b)3 1/3y+16 3/4=-13,25`
`⇔10/3y+67/4=-53/4`
`⇔10/3y=-30`
`⇔y=-9`
Vậy `y=-9`
`c)(\frac{3x}{7}+1):(-4)=(-1/28)`
`⇔\frac{3x}{7}+1=-1/28.(-4)`
`⇔\frac{3x}{7}+1=1/7`
`⇔\frac{3x}{7}=-6/7`
`⇔3x.7=-6.7`
`⇔21x=-42`
`⇔x=-2`
Vậy `x=-2`