`a)`
Cách `1 :`
`1/3 : 2/5 + 2/3 : 2/5`
`= 1/3 xx 5/2 + 2/3 xx 5/2`
`= 5/6 + 10/6`
`= 15/6`
`= 5/2`
Cách `2 :`
`1/3 : 2/5 + 2/3 : 2/5`
`= ( 1/3 + 2/3 ) : 2/5`
`= 3/3 xx 5/2`
`= 1xx 5/2`
`= 5/2`
`b)`
Cách `1 :`
`5/6 : 7/8 - 1/4 : 7/8`
`= 5/6 xx 8/7 - 1/4 xx 8/7`
`= 40/42 - 8/28`
`= 20/21 - 2/7`
`= 20/21 - 6/21`
`= 14/21`
Cách `2 :`
`5/6 : 7/8 - 1/4 : 7/8`
`= ( 5/6 - 1/4 ) : 7/8`
`= 7/12 xx 8/7`
`= 8/12`
`= 2/3`