Đáp án:

 `a)x=10/9`

`b)x=2`

Giải thích các bước giải:

 `a)`

`x xx 2/7 xx 3/4=5/21`

`x xx (2/7xx 3/4)=5/21`

`x xx (\frac{2xx3}{7xx2xx2})=5/21`

`x xx 3/14=5/21`

`x=5/21: 3/14`

`x=5/21 xx 14/3`

`x=\frac{5xx2xx7}{3xx7xx3}`

`x=\frac{10}{9}`

Vậy `x=10/9`

`b)`

`x:4/5=25/8:5/4`

`x:4/5=25/8 xx 4/5`

`x:4/5=\frac{5xx5xx4}{2xx4xx5}`

`x:4/5=\frac{5}{2}`

`x=5/2 xx 4/5`

`x=\frac{5xx2xx2}{2xx5}`

`x=2`

Vậy `x=2`