Đáp án:
`3/7`
Giải thích các bước giải:
`3/7 . ( 13/8 - 7/9 ) - 5/8 . ( 3/7 - 8/15 )`
`= 3/7 . 13/8 - 3/7 . 7/9 - 5/8 . 3/7 + 5/8 . 8/15`
`= ( 3/7 . 13/8 - 5/8 . 3/7 ) - 3/7 . 7/9 + 5/8 . 8/15`
`= 3/7 . ( 13/8 - 5/8 ) - 1/3 + 1/3`
`= 3/7 . 8/8 = 3/7 . 1 = 3/7`