405. Find all natural numbers, when divided by 6, the quotient is the same as the remainder.
$\triangle$ Let the desired number be denoted by $a$, and the quotient and remainder simultaneously by $q$. Then

$$
a=6 q+q=7 q
$$

It might seem that the answer includes all natural numbers divisible by 7. However, this is not the case, since the remainder $q$ must satisfy the inequality $0<q<6$. By setting $q=1,2,3,4,$ and 5, we find all possible values of $a$.

Answer: $7,14,21,28,35$.