Problem 9 · 2012 Math Kangaroo
Medium
Arithmetic & Operations
careful-countingoff-by-one
The last row in an aeroplane is row 25. There is no row 13, and row 15 has only 4 seats. Every other row has 6 seats. How many passenger seats are there on this aeroplane?
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Answer: C — 142
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Hint 1 of 2
First count how many rows actually exist (watch out for the missing row 13).
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Hint 2 of 2
Most rows have 6 seats; only row 15 is different, so adjust for it.
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Approach: count rows, then adjust the odd one out
- Rows are numbered 1 to 25 but row 13 is missing, so there are 24 rows.
- If every row had 6 seats that would be 24 x 6 = 144.
- Row 15 has only 4 instead of 6, which is 2 fewer, so 144 - 2 = 142 seats.
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