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Answer: 81 seconds
You can count out 9 numbers and add them up:
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.
Or use patterns:
Street Lamps
Age 11 to 14
ShortChallenge Level 
Answer: 81 seconds
You can count out 9 numbers and add them up:
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.
Or use patterns:
This problem is taken from the UKMT Mathematical Challenges.
You can find more short problems, arranged by curriculum topic, in our short problems collection.
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