20. Find the smallest natural number divisible by 72, in the representation of which all digits from 1 to 9 appear.
20. The desired number must be divisible by 72, which means it must be divisible by 9 and 8; $1+2+\ldots+9=45$, which is divisible by 9. For the number to be divisible by 8, it must end with three digits that form a three-digit number divisible by 8. Considering all this, the digits should be arranged so that the smallest ones are to the left. Such a number is 123457968.