80*79分之7+78*79分之5
=（79+1）/7/79+（79-1）*5/79
=79*7/79+7/79+79*5/79-5/79
=7+7/79+5-5/79
=＝12又2/79
5分之7*9分之4-5分之2*9分之4
=（7/5-2/5）*4/9
=1*4/9
=4/9