Answer: For a fence that has 18 posts he needs 36 rails.
Explanation:
Hi, to answer this question we have to use proportions:
Each section use 3 vertical fence post and 6 horizontal rails
The proportion is = 3 post /6 rails
For 18 posts:
18 posts / x rails.
3 /6 = 18 /x
Solving for x:
x = 18 / (3/6)
x = 36 rails
For a fence that has 18 posts he needs 36 rails.
Feel free to ask for more if needed or if you did not understand something.