4.2*101
=4.2x(100+1)
=4.2x100+4.2
=420+4.2
=424.2
1/10*99+1/10
=1/10x(99+1)
=1/10x100
=10
24*（1/4+1/6-1/3）
=24x1/4+24x1/6-24x1/3
=6+4-8
=2