Half of what Marco has is $24 / 2 = $<<24/2=12>>12.
If he gives Mary $12, she has $12 + $15 = $<<12+15=27>>27.
Now subtract the money Mary spends: $27 - $5 = $<<27-5=22>>22.
Now subtract Marco's money from Mary's remaining money, $22 - $12 = $<<22-12=10>>10.
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