16.8-5.1-4.9
=16.8-(5.1+4.9)
=16.8-10
=6.8
14.2*99+14.2
=14.2*(99+1)
=14.2*100=1420
（1/4+5/6-1/3）*12
=(1/4)*12+(5/6)*12-(1/3)*12
=3+10-4
=9