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Make a set of numbers that use all the digits from 1 to 9, once and once only. Add them up. The result is divisible by 9. Add each of the digits in the new number. What is their sum? Now try some other possibilities for yourself!

### Double Digit

The number $12 = 2^2 \times 3$ has 6 factors. What is the smallest natural number with exactly 36 factors?