4÷2/3×7/12
=4×3/2×7/12
=3.5
8/9÷4/27+3/7
=6+3/7
=6又3/7
8/13÷3/7+5/13÷3/7
=8/13×7/3+5/13×7/3
=（8/13+5/13）×7/3
=1×7/3
=2又1/3