Find all the five-digit numbers which divisible by 3 and are formed using the digits 1, 6 All I have done is that the sum of each digits of the five digit number should be divisible by 3. So $$a_1 + a_2 + a_3 + a_4 + a_5 = 3x$$
Can go as far as that

If the number has $a$ 1s and $b$ 6s, then $$a+b=5 \text { and } 3 \text { divides } a+6b.$$
So either
$a=0, b=5$ and the number is 55555
or
$a=3, b=2$ and the numbers are the  $\begin{pmatrix}5\\3\\\end{pmatrix}=10 $ permutations of 11166.
