(4)2/7÷8/21
=2/7×21/8
=1×3/4
=3/4
(5)2/9×3/8÷6/7
=2/9×3/8×7/6
=1/3×1/4×7/6
=7/72
(6)4÷8/3-3/5
=4×3/8-3/5
=1×3/2-3/5
=3/2-3/5
=15/10-6/10
=9/10
(7)3/4×5/6+3/4×1/6
=3/4(5/6+1/6)
=3/4×1
=3/4
4,四分之三
5,72分之7
6,10分之9
7,四分之三