`A= 3/4 xx 2/5 + 3/8 xx 7/13 + 3/5 xx 3/4 + 3/13 xx 6/8`
`A= 3/4 xx ( 2/5 + 3/5) + 3/8 xx 7/13 + 3/13 xx 3/4`
`A= 3/4 xx . 1 + 3/4 xx 3/13 + 7/13 xx 3/8`
`A= 3/4 ( 1 + 3/13) + 7/13 xx 3/8`
`A= 3/4 xx 16/13 + 7/13 xx 3/8`
`A= 12/13 + 7/13 xx 3/8`
`A=12/13 + 21/104`
`A= 9/8`
Vậy `A= 9/8`