Tìm y biết: 5/12 x (8+y) - 1/5 x (15/4 + y) = 15

`5/12 × (8 + y) - 1/5 × (15/4 + y) = 15`

`= 5/12 × 8 + 5/12y - 1/5 (15/4 + y) = 15`

`=` ${ \dfrac{5 × 8}{12}}$ `+ 5/12y - 1/5 (15/4 + y) = 15`

`= 40/12 + 5/12y - 1/5 (15/4 + y) = 15`

`= 10/3 + 5/12y - 1/5 (15/4 + y) = 15`

`= 10/3 + 5/12y - 1/5 × (15/4) - 1/5y = 15`

`= 10/3 + 5/12y +` ${ \dfrac{- 15}{5 × 4}}$ `- 1/5y = 15`

`= 10/3 + 5/12y +` ${ \dfrac{-15}{20}}$ `- 1/5y = 15`

`= 10/3 + 5/12y - 3/4 - 1/5y = 15`

`= 40/12 + 5/12y - 9/12 - 1/5y = 15`

`=` ${ \dfrac{40 - 9}{12}}$ `+ 5/12y - 1/5y = 15`

`= 31/12 + 5/12y - 1/5y = 15`

`= 31/12 + 13/60y = 15`

`= 13/60y = 15 - 31/12`

`= 13/60y = 180/12 - 31/12`

`= 13/60y =` ${ \dfrac{180 - 31}{12}}$

`= 13/60y = 149/12`

`y = 149/12 × (60/13)`

`y =` ${ \dfrac{149 × 60}{12 × 13}}$

`y = 8940/156`

`y = 745/13`

Cho mình xin hay nhất nha bạn!!!!!

@Drew Mclntyre