Henry went to the beach and picked up 17 perfect shells and 52 broken shells. Out of those 52 broken shells, half of them were spiral. Out of the 17 perfect shells, 12 of them were not spiral. How many more broken spiral shells were there than perfect spiral shells? Let's think first. Chain of thought:
52 broken shells / 2 = 26 broken shells were spiral. 17 perfect shells - 12 not spiral perfect shells = 5 perfect shells were spiral. There were 26 - 5 = 21 more broken spiral shells than perfect spiral shells.
Therefore, the answer is 21.