If the number is odd, [tex] u [/tex] can only be 1, 3, 5, 7 or 9.
If the number is less than 30, [tex] t [/tex] can be 1 or 2.
The sum of the digits has to be 10, so [tex] t [/tex] can't be 2, because it would lead to the number 28, which is even.
So, [tex] t [/tex] has to be 1, which implies that [tex] u=9 [/tex]
In fact, the number 19:
- is odd
- the unit digit is greater than the tens digit ([tex] 9>1 [/tex])
- is less than 30
- the sum of its digits is 10
