Question 176296
You're right, you need to know at least one more piece of information.  All you can do now is say that Mary makes m.  Then Jack makes 2m and Jill makes m+8.  But without a given constant, you're stuck.  You can however, give conditions:
When do they make the same?
2m=m+8
So m=8.  If Mary makes $8, Jack and Jill make the same amount.  If Mary makes less than $8, say $7 for an example, Jack makes $14 and Jill makes $15, so Jill will be the winner if Mary makes less than $8.  If Mary makes more than $8, say $9 for an example, Jack makes $18 and Jill makes $17.  So if Mary makes more than $8, Jack is the winner.  
Unfortunately, the creator of the aptitude test did not consult a mathematician :)