Question

This mystery number has 3 digits.Its digits are in the descending order. The hundreds' digit is two more than the ones' digit. If you add the 3 digit it gives you a dozen. Guess this mystery number?

Answer 1

If the digits are in strict descending order, then they must be [n+1]n[n-1], the sum of which is 12, i.e.
n+1 + n + n-1 = 12
Solve for n.

Answer 2

The hundred digit is two more than the ones.
If you add three it will be 12.
Which means the hundreds is 12-3=9
To find the ones 9 - 2=7
Now we found 9_7.
To finish for sure the decending means 8
So the answer is 987.