When it comes to problems like this, I usually look through the answers and try to plug in the numbers until I get the right equation that fits with the graph.
2.5(5) = 12.5
2.5(10)= 25
2.5(15)= 37.5
2.5(20) = 50
2.5(25) = 62.5
If you compare this to the graph, you'll notice the solutions are the same as the Total Cost (c). That's because c= 2.5d is the right equation for this graph. (Sorry if my explanation is confusing)
