`2)`
`a) (-25)/28 . 21/100`
`= \frac{-25 . 21}{28 . 100}`
`= (-1 . 3}{ 4 . 4}`
`= -3/16`
``
`b) 7/9 : (-35)/12`
`= 7/9 . (-12)/35`
`= \frac{7 . -12}{9 . 35}`
`=\frac{1 . -4}{3 . 5}`
`= -4/15`
``
`3`
`a) 5/31 . 21/25 + 5/31 . (-7)/10 - 5/31 . 9/20`
`= 5/31 . ( 21/25 - 7/10 - 9/20)`
`= 5/31 . (-31/100)`
`= -1/20`
``
`b) ( 13/24 - 29/30) : (-51/5)`
`=( 65/120 - 116/120) . (-5/51)`
`=-17/40 . (-5/51)`
`= 1/24`
``
`4)`
`a) 10/9 : x + 7/12 = -2/3`
`10/9 : x = -2/3 - 7/12`
`10/9 : x = -5/4`
`x = 10/9 : (-5/4)`
`x = -8/9`
Vậy `x = -8/9`
``
`b) ( 4/15 - 1/6) . x = -3/4 . 8/27`
`1/10 . x = -2/9`
`x = -2/9 : 1/10`
`x = -20/9`
Vậy `x = -20/9`
