Question 917967
I will need at least 3 digits because {{{8+9=17<21}}} is the greatest number than I can make with just 2 different digits.
Since the same digit has a value 10 times greater than the same digit to its right,
we want to make the first and second digits as small as possible, at the expense of increasing the digits to their right.
As a consequence we would make the last digits 89,
with {{{8+9=17}}} ,which would require a {{{21-17=4}}} for a hundreds digit.
So, the smallest whole number you can make whose digits are all different numbers and the sum of the digits is 21, is {{{highlight(489)}}} .