Marco has $24. Mary has $15. If Marco gives Mary half of what he has, Mary will have more than Marco. But Mary spends $5. How much more money does Mary have than Marco? Let's be accurate as possible.
Half of what Marco has is $24 / 2 = $12. If he gives Mary $12, she has $12 + $15 = $27. Now subtract the money Mary spends: $27 - $5 = $22. Now subtract Marco's money from Mary's remaining money, $22 - $12 = $10.
The answer: 10.