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Puzzle 0001 - Legs in the bus
Correct answer:
7 kids * 2 legs = 14 legs
7 kids * 7 rucksacks = 49 rucksacks
49 rucksacks * 7 big cats * 4 legs = 1,372 legs
49 rucksacks * 7 big cats * 7 small cats * 4 legs = 9,604 legs
14 + 1,372 + 9,604 = 10,990 legs total.

This may also be shown as a simple powers of 7 problem:

Total legs = 7^1 * 2 + 7^2 * 0 + 7^3 * 4 + 7^4 * 4

This is because there are:
7^1 * 2 kids legs
7^2 rucksacks (But they don't have legs)
7^3 * 4 big cat legs
7^4 * 4 small cat legs

Of course, 7^2 * 0 could be omitted completely, but I include it just to show the progression of powers of 7.

Totals:

Kids: 7
Rucksacks: 49
Big cats: 343
Small cats: 2,401
Cats per rucksack: 56


Top 5 incorrect answers:

Rank 1
Answer: 1,582
Seen: 26,170 times.
This was the most frequent incorrect answer by far. The problem here was that only 7 rucksacks were considered and not the proper 49.

How this incorrect answer was calculated:
7 kids * 2 legs = 14 legs
7 rucksacks * 7 big cats * 4 legs = 196 legs
7 rucksacks * 7 big cats * 7 small cats * 4 legs = 1,372 legs
14 + 196 + 1,372 = 1,582 legs total.

Rank 2
Answer: 238
Seen: 16,803 times.
It seems in this answer that there has only been 1 or no rucksacks considered. If this were changed to 49, the answer would have been correct.

How this incorrect answer was calculated:
7 kids * 2 legs = 14 legs
1 rucksack * 7 big cats * 4 legs = 28 legs
1 rucksack * 7 big cats * 7 small cats * 4 legs = 196 legs
14 + 28 + 196 = 238 legs total.

Rank 3
Answer: 9,618
Seen: 16,509 times.
This answer appears to incorrectly calculate the total number of cats in a single sack to be 49, when there are in fact 56, or more simply the big cat legs have not been summed in the final calculation.

How this incorrect answer was calculated:
7 kids * 2 legs = 14 legs
7 kids * 7 rucksacks = 49 rucksacks
49 rucksacks * 7 big cats * 7 small cats * 4 = 9,604 legs
14 + 9,604 = 9,618 legs total.

Rank 4
Answer: 14
Seen: 15,299 times.
Generally the logic in this answer is something along the lines of "Only the children are in the bus, the cats are in rucksacks". However, everything is clearly in the bus.

How this incorrect answer was calculated:
7 kids * 2 legs = 14 legs total.

Rank 5
Answer 1,386
Seen: 13,421 times.
This answer was achieved by forgetting about the small cats. If you add the small cat legs to this answer you arrive at the correct answer.

How this incorrect answer was calulated:
7 kids * 2 legs = 14 legs
7 kids * 7 rucksacks * 7 big cats * 4 = 1,372 legs
14 + 1,372 = 1,386 legs total.

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