`4.`
`a)`
`5``4/7 : x = 13`
`x= 5 frac{4}{7} : 13`
`x=39/7 . 1/13`
`x=3/7`
Vậy `x=3/7`
`b)`
`2/3x-1/2x=15/12`
`=>(2/3-1/2)x=15/12`
`=>1/6x=15/12`
`=>x=15/12:1/6`
`=>x=15/2`
Vậy `x=15/2`
`5.`
`B=(6-2 4/5).3 1/8 - 1 3/5 : 1/4`
`=` `(6-14/5).25/8-8/5:1/4`
`=` `frac{16}{5}.frac{25}{8}-8/5:1/4`
`=` `10-8/5.4`
`=` `10-32/5`
`=` `18/5`