$n!=n(n-1)(n-2)\cdots 3\times 2\times 1 $ now let $n!=n(n-1)(n-2)\cdots 3\times 2\times 1 $
now let
$5!m!=n! , m\geq 4$ then $n+m=?$
My Try :
case$1$ Let $n=m$:
$$5!m!=m! \\ 5! \neq 1$$ so $n\neq m$
case$2$ Let $n>m$  :
$$5!=\dfrac{n!}{m!} \\ 5!=n(n-1)(n-1) . . . (n-m)$$
case$3$ Let $m>n$
$$5!m!=n! \\ m(m-1)(m-2)...(m-n)=\dfrac{1}{5!}=\dfrac{1}{120}$$
since $\dfrac{1}{120}$ is't natural number so $n>m$
Now what ?

You can just write $5!=120=\frac{n!}{m!}=n(n-1)\cdots(m+1).$
Now just divide in cases: if $n-m=1$ then $n=120$, and by consequence $m=119.$
If $n-m=2$ we should write $120$ as $n(n-1),$ but this is impossible (cause $10\times11=110<120$ and $11\times12=132>120$). 
If $n-m=3$ the solution is given by $120=6\times5\times4$ so that we get $n=6, m=3.$
If $n-m=4$ then $n(n-1)\cdots(n-3)=120$ which implies $n=5,$ and $m=1.$
Same as the last one, if $n-m=5$ then $n(n-1)\cdots(n-4)=120$ which implies $n=5,$ and $m=0.$
The difference can't be bigger because there would be too many factors to obtain $120.$
