If three distinct integers are chosen at random, show that there will exist two among them, say $a$ and $b$, such that $30 | (a^3b-ab^3)$ Problem:

If three distinct integers are chosen at random, show that there will exist two among them, say $a$ and $b$, such that $30 | (a^3b-ab^3)$

My work:
$a^3b-ab^3=ab(a+b) (a-b)$ and if $30 | (a^3b-ab^3)$, then each of $2,3,5$ divides $(a^3b-ab^3)$ since $2\cdot3\cdot5=30$ and $(2,3,5)=1$
Case for 2:
If 2 divides either $a$ or $b$, then all good.
If not, then $a$ and $b$ are both odd and so $a+b$=sum of odds = even is divisible by 2. Done.
Case for 3:
If 3 divides a or b, good.
Else, let:

*

*$a=3k+1$ and $b=3k+1$. Then, $3|(a-b)$

*$a=3k+1$ and $b=3k+2$. Then, $3|(a+b)$

*$a=3k+2$ and $b=3k+2$. Then, $3|(a-b)$

*$a=3k+2$ and $b=3k+1$. Then, $3|(a+b)$
So far, so good.
Case for 5:
Completely stuck here. Five possible remainders ($0,1,2,3,4$) seem to be impossible to do. Any hints?
Questions:

*

*Need hints for last case.


*Why do we need to pick three integers when we only need to check with two?

Let $p,q$ be remainders when $5$ divides $a,b$ respectively. If any of the following holds:


*

*$p=0$

*$q=0$

*$p=q$

*$p+q = 5$


Then, $5|a$, $5|b$, $5|(a-b)$ or $5|(a+b)$ respectively.
So, now check for $(p,q)=$


*

*$(1,2)$ Now, if $c = 5k+1$, then $5|(a-c)$, so repeat the question taking the two numbers $a,c$ noting that these two already answer the case for $5$. If, $c=5k+2$, then $5|(b-c)$, proceed similarly as above.  If $c=5k+3$, then $5|(b+c)$, proceed similarly as above. If $c=5k+4$, then $5|(a+c)$, proceed similarly as above. If $c=5k$, then $5|c$ so take the numbers $a,c$ or $b,c$ and proceed similar to above.

*For the remaining tuples, repeat similar to what we did in point 1 just above.
Q.E.D.
