So you have 6 numbers, ranging from 1-24. Five of the six numbers must equal the 6th number by using any of the 4 operations (+, -, *, /) and using each number only once.

So for example:

3 7 12 19 21 | 5

So you need to get the number 5 by moving around and using the operations on those five first numbers.

Now, find an example where this is impossible!

EDIT: I think I found one:

5 11 17 19 23 -> 1

See if you can find away this would work, cause I cannot!

Also, how would one go about proving it is a valid counter example?