Fence Posts

Steve is helping his dad to build a fence (pictured below). Between each post they are going to put 3 horizontal bars.

If they build a fence that has 5 posts how many bars are needed? 6 posts? 7 posts? 20 posts?

Write a number sentence that expresses the number of bars in terms of the number of posts in the fence.

Adapted from Balanced Assessment in Mathematics

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Arranging Toothpicks