78*10.2
=（80-2）*10.2
=80*10.2-2*10.2
=102*8-2*10.2
=816-20.4
=816-16-4.4
=800-4.4
=795.6
或
78*10.2
=78*（10+0.2）
=78*10+78*0.2
=780+15.6
=795.6