$a)$
`5/7 . 5/11 + 5/7 . 8/11 - 5/7 . 2/11`
`= 5/7 ( 5/11 + 8/11 - 2/11)`
`= 5/7 . 11/11`
`= 5/7 . 1`
`= 5/7`
$b)$
`4 2/7 - 3 6/13 - 21/11 + 1 5/7 + 5 6/13`
`= ( 4 2/7 + 1 5/7 ) + (- 3 6/13 + 5 6/13) + -2 1/11`
`= 6 + 2 + - 2 1/11`
`= 8 - 23/11`
`= 88/11 - 23/11`
`= 65/11`
Tìm `x`
`1/6 - 75% x = 0,2`
`1/6 - 3/4 x = 1/5`
`3/4 x = 1/6 - 1/5`
`3/4x = 5/30 - 6/30`
`3/4x = -1/30`
`x = -1/30 . 4/3`
`x = -4/90`
`x= -2/45`