All (3) of the triangles are similar to one another, and they have the 30-60-90 interior-angle pattern. This allows us to establish the following relations.
I'm pretty sure if you divide 3.75 by 3 you will get the amount of $ for a bag of soil which is around $1.25/per bag. And then if you multiple $1.25 for 7 bags you will get $8.75
