（1－1/12×3）÷1/6
=（1－1/12×3）×6
=1×6-1/4×6
=6-3/2
=9/2
5/7×[1－（1/4＋2/5）]
=5/7×【3/4-2/5】
=5/7×7/20
=5/20
=1/4