64.2*87+0.642*1300的简便运算怎么做啊?

64.2*87+64.2*13
=64.2*(87+13)
=6420

64.2*87+0.642*1300
=64.2*87+0.642*(13*100)
=64.2*87+0.642*100*13
=64.2*87+(0.642*100)*13
=64.2*87+64.2*13
=64.2*(87+13)
=64.2*100
=6420

64.2*87+0.642*1300
=64.2*87+64.2*13
=64.2*(87+13)
=64.2*100
=6420