The product of digits equal to the sum of digits How to find the number(or numbers ) that has $4$ digits, the product of these digits equal to the sum of these digits ?
First of all, let's observe that all of the digits of such a number cannot be the same. You can just manually check that numbers $1111$, $2222$ and so on don't suit us. It is also clear that all of the digits should be non-zero.

Now suppose that we have such a number. Let $a,\,b,\,c,\,d$ be its digits written in non-ascending order: $a \geqslant b \geqslant c \geqslant d$. Then we have
$$
abcd = a + b + c + d.
$$

From this we have an inequality:
$$
a\cdot bcd &lt; 4a.
$$
This inequality is strict, because at least one of $b, c, d$ is strictly smaller than a. So we have:
$$
bcd &lt; 4,
$$
which is the same as saying
$$
bcd \leqslant 3.
$$
This only leaves us with 3 possible combinations for $(b, c, d)$: $(1, 1, 1)$, $(2, 1, 1)$ and $(3, 1, 1)$.

If $b=c=d=1$, then $a\cdot 1 \cdot 1 \cdot 1 = a + 1 + 1 + 1$, which can't be true.

If $b=2$ and $c=d=1$, then $a \cdot 2 \cdot 1 \cdot 1 = a + 2 + 1 + 1$, which means that $a=4$. This gives us one possible solution: $a=4, b=2, c=d=1$.

If $b=3$ and $c=d=1$, then $a \cdot 3 \cdot 1 \cdot 1 = a + 3 + 1 + 1$, which is impossible.

So, the only solution is $a=4$, $b=2$, $c=d=1$. There are $12$ numbers with such digits.
Must be various combinations of $1,1,2,4$.
I don't think there are any other combinations, I looked at all small numbers...