This feels like we are trying to write a formula for a word problem without seeing the actual word problem.
There are several different formulas that will produce the number that you want. For example, the following formula will also produce 13.33:
({Product Length} / {Material Length}) * {Purchase Cost}
However, you should use a formula that best represents the original word problem. This will make maintenance easier later on.
Without knowing more about the original problem, it looks like you have purchased 48" inches of some material for $39.99. You want to use 16" of that material to create a product. You want calculate the cost of your product, so you need to see how much the 16" costed.
With that in mind, I would use the following formula which multiplies the amount of material that you are using times the unit cost of the material.
{Product Length} * ( {Purchase Cost} / {Material Length} )
Notice that all three of the formulas (@Bill.French’s formula and the two I present in this post) will all product the exact same result. You are free to use whichever one makes the most sense to you and will be the easiest to maintain. If I am totally wrong about your original word problem and none of the formulas make sense to you, feel free to post it and we can see if we can come up with a different formula that makes sense to you.
