AMO 2018 – Secondary 3 (Grade 9) Question 20

Define $n!=n(n-1)\cdots2\cdot1$; for example $4!=24$ and $3!=6$. Simplify:

$$\frac{81!+80!\times70-80!}{20\times79!+15\times79!+10\times79!+5\times79!}$$