Q:

$a!b!$ multiple of $a! + b!$ implies $3a\geq 2b + 2$

A colleague asked me to show that if $a,b\in\mathbb{N}^*$ are such that $a!b!$  is a multiple of $a! + b!$, then $3a\geq 2b + 2$.
While I could verify this inequality numerically for many solutions $(a,b)$, I did not find anything relevant mathematically. I must say I have never faced such inequalities in arithmetic... 
The problem is symmetric in $a$ and $b$, and actually I could see that the solutions where all such that $3a\geq 2b + 2$ and $3b\geq 2a + 2$.
Here is a plot of the first solutions $(a,b)$ and the bounds:

The solutions can be generated with the Mathematica code:
Table[If[IntegerQ[a!*b!/(a! + b!)], {a, b}, 
 Unevaluated[Sequence[]]], {a, 1, 100}, {b, 2, 100}]

A:

Assume wlog. that  $b\ge a$. Then both $a!b!$ and $a!+b!$ are divisible by $a!$ and $a!+b!\mid a!b!$ implies $1+\frac{b!}{a!}\mid b!$. For $a<k\le b$, $1+\frac{b!}{a!}$ is coprime to $k$. We conclude that $1+\frac{b!}{a!}\mid a!$.
For $1\le k\le  b-a$, $k$ has a multiple $qk$ with $a<qk\le b$, hence is again coprime to $1+\frac{b!}{a!}$. We conclude $1+\frac{b!}{a!}\mid \frac{a!}{(b-a) !}$, in particular $$\tag1\frac{b!}{a!}< \frac{a!}{(b-a) !}.$$
By comparing the not cancelled factors in $(1)$ with $a$, we obtain
$$ a^{b-a}<a^{2a-b}$$
and hence (for $a>1$)
$$\tag2 2b<3a.$$
To improve this, observe that one of $b-a+1, b-a+2$ is not prime (or we are in the trivial case $b=a+1$), hence can be written as product of numbers $<b-a$ and is once again coprime to $1+\frac{b!}{a!}$. This allows us do divide off one more factor on the right of $(1)$, thus leading to $2b<3a-1$ and hence
$$ 2b+2\le 3a.$$

A:

This problem is $2015$ IMO Shortlist Number theory N$2$. Two solutions can be found here on page $66$.
Some solutions can be found, e.g., here on Art of Problem Solving.

