-1/4+7/33-5/3-(_15/12+6/11-68/49)

`-1/4+7/33-5/3-(-15/12+6/11-68/49)`

`=-1/4+7/33-5/3+5/4-6/11+68/49`

`=(-1/4+5/4)+(7/33-6/11-5/3)+68/49`

`=1-2+68/49`

`=-1+68/49`

`=19/49`

 ___

`a)`

`4/7 . x-2/3=1/5`

`<=>4/7 . x=1/5+2/3`

`<=>4/7 . x=13/15`

`<=>x=13/15:4/7`

`<=>x=91/60`

Vậy `x=91/60`

`b)`

`x+8,5=21,7`

`<=>x=21,7-8,5`

`<=>x=66/5=13,2`

Vậy `x=13,2`

`@riiumecou`