Draw 5 blanks for the 5 digits:
the last is one half the second and one third the first
That narrows it to these three possibilities:
3 2 1
6 4 2
9 6 3
the first two digits are the square of the third
The only one of those three that has the first two digits
forming a square is
6 4 2
since 64 is the square of 8. So the third digit is 8.
6 4 8 2
the last two digits are the sum of the second and third
4+8=12 so the last two digits is 12, which means the
fourth digit is 1
6 4 8 1 2
That's the answer!
We didn't need to use:
the third is double the second,
But it checks since 8 is indeed double 4
We also didn't need to use
The sum of all the digits is 21.
But it checks also since 6+4+8+1+2 = 21
Edwin