Fence Posts
Read the puzzle
A straight fence is 15 m long. It has a post every 3 m, with one post at each end. A small example shows a 6 m fence with posts numbered 1, 2 and 3 and two 3 m gaps. How many posts does the 15 m fence have?
A straight 15 m fence has a post every 3 m, with one at each end. How many posts are there?
Count the gaps first, then the posts.
6
15 ÷ 3 = 5 gaps, and 5 gaps need 6 posts.
How to solve it
Count gaps first, then posts. Posts 3 m apart along 15 m make 15 ÷ 3 = 5 gaps. On a straight fence with a post at each end, the first post starts the fence and every gap ends with a new one, so there is one more post than gaps: 5 + 1 = 6. The small example agrees: 6 m gives 2 gaps and 3 posts.
The usual wrong answer is 5, which counts gaps and leaves one end bare.
Quick check: posts stand at 0, 3, 6, 9, 12 and 15 m, which is 6.
Learn the method: Fence Post Riddle: Why a 15 m Fence Needs 6 Posts
