（5又4/15-（0.25+1/3）*7.2）/2又2/5
=（5又4/15-0.25*7.2-1/3*7.2）/2又2/5
=（5又4/15-1.8-2.4）/2又2/5
=（5又4/15-4又3/15）/12/5
=16/15×5/12
=4/9