Problem 7 · 2013 Math Kangaroo
Medium
Arithmetic & Operations
arithmetic-sequenceoff-by-one
Michael must take a tablet every 15 minutes. He takes the first one at 11:05. When does he take the fourth?
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Answer: B — 11:50
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Hint 1 of 3
Count the waits between tablets, not the tablets themselves.
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Hint 2 of 3
From the 1st to the 4th tablet there are only 3 waits, each 15 minutes long.
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Hint 3 of 3
Add those three 15-minute waits to the 11:05 start time.
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Approach: count the gaps, not the tablets
- Going from the 1st tablet to the 4th passes 3 gaps (1st→2nd, 2nd→3rd, 3rd→4th), each 15 minutes.
- That is \(3 \times 15 = 45\) minutes of waiting in all.
- Starting at 11:05 and adding 45 minutes gives 11:50, which is choice B.
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