Street Lamps
Walking up a steep hill, I pass 10 equally spaced street lamps. How long do I take to walk from the first lamp to the last?
Age
11 to 14
Challenge level
Walking up a steep hill, I pass 10 equally spaced street lamps.
I take 5 seconds to walk from the first lamp to the second lamp, 6 seconds from the second lamp to the third, and so on, with each time increasing by 1 second as I slow down.
How long do I take to walk from the first lamp to the last?
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.
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:
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