21×【1＋1/7】
=21×1+21×1/7
=21+3
=24
3/7÷3-1/21=
=1/7-1/21
=2/21
5/9× 6/25÷3/10
=5/9×6/25×10/3
=4/9
1/9×24＋24×8/9
=24×(1/9+8/9)
=24×1
=24