$a$) `(x + 3/5) : 4/3 = 5/6`
`⇔ x+ 3/5 = 10/9`
`⇔ x = {23}/45`
Vậy `x = 23/45`
$b$) `x. 1212/1515-x. 1919/2020=6/-5`
`⇔ x (12/15 - 19/20) = -6/5`
`⇔ x . {-3}/20 = -6/5`
`⇔ x = 8`
Vậy `x=8`
$a$) `-17/18. 3/7-17/18 .4/7`
`= -17/18 . (3/7 + 4/7)`
`= -17/18`
$b$) `7777/8888. 13/17+7/8. 4/17`
`= 7/8 . (13/17 + 4/17)`
`=7/8`
$a$) `(-3/4+2/5):4/15+(3/4-4/5).5/2`
`= -21/16 - 1/8`
`= {-23}/16`