4.2/3又2分之1-2又5分之2*4分之1
=4.2/3.5-2.4×1/4
=1.2-0.6
=0.6
7分之2*（4.9-2又10分之1）+20分之9/2.25
=4.9×2/7-2.1×2/7+0.45/2.25
=1.4-0.6+0.2
=1.6-0.6
=1