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Weekly Problem 9 - 2007

Stage: 2 and 3 Challenge Level: Challenge Level:1
81s.

Note that there are 9 gaps between the lamps, so the total time is
(5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13) seconds = 81 seconds.

This problem is taken from the UKMT Mathematical Challenges.

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