~ Bạn tham khảo ~
`a, 1/3 xx 4/5 + 1/3 xx 6/5 + 2/3`
`= 1/3 xx ( 4/5 + 6/5) + 2/3`
`= 1/3 xx 10/5 + 2/3`
`= 1/3 xx 2 + 2/3`
`= 2/3 + 2/3`
`=4/3`
`b, 5/21 + ( 16/21 + 1 )`
`= 5/21 + 1 16/21`
`= 1 + ( 5/21 + 16/21 )`
`= 1 + 21/21`
`= 1 + 1`
`=2`
`c, ( 7/20 + 11/15 - 15/12 ) : ( 11/20 - 2/10 )`
`= ( 21/60 + 44/60 - 5/4 ) : ( 11/20 - 4/20 )`
`= ( 65/60 - 5/4 ) : 7/20`
`= ( 65/60 - 75/60 ) xx 20/7`
`= -10/60 xx 20/7`
`= -1/6 xx 20/7`
`= -20/42`
`= -10/21`