Off by One: Fence Posts and Gaps

Methods in this video

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1. 1

   Count the gaps, then check the ends

   ▶ 0:48

   When to use it: Things are set out at equal gaps (posts, lines, lights, days, a list of numbers). You know the length and the gap and need how many there are, or you know how many and need the length or where the nth one is. Look for apart, every, from … to …, the first … the last.

   • Dividing gives you the gaps, but the question may ask for the posts: work out what your number is counting.
   • On a straight line with a post at both ends, there is one more post than gaps; n posts have only n − 1 gaps.
   • Around a closed loop there are no ends, so posts and gaps are equal; adding one per side counts each corner twice.
   • Days and lists of numbers work like posts: 29 − 12 tells you the gap, so if both ends count, add one.