Answer:
Diego is correct, he should move back one space.
Step-by-step explanation:
If every 5th customer gets a free bagel then you should be able to count up by 5s to figure out which people get the bagel. (5, 10, 15, etc.)
It would be the same for 9 and 12 (9, 18, 27, etc.) (12, 24, 36, etc.).
However, 23 is not divisible by 5, 9, or 12 (I moved on to division because it's the fast way to check)
But if you look at the multiples of 12 then you'll see that the 24th person would get a prize.
So, Diego should go back one space in order to get a prize because 23 is not a multiple of 5, 9, or 12. However, 24 is a multiple of 12.
Hope this helps and makes sense.