（1/3+1/4)x6/7
=7/12×6/7
=1/2
1/2÷3/4+3/8
=1/2×4/3+3/8
=2/3+3/8
=(16+9)/24
=25/24
2/3x5/7+2/7÷3/2
=10/21+2/7×2/3
=10/21+4/21
=14/21
=2/3