I woke up this morning and wondered if we had been slightly miskeading you about the graph. As experienced people we all spotted that it would be a straight-line graph; as a student you would not necessarily spot that. What you have to do is to draw a graph with b along the x-axis and c up the y-axis. You then plot every point on the graph ie, (1,24), (2,48) etc and then look to see what sort of line you can connect them with. You will see that it is a straight line, which happens to go through the origin because zero cases contain zero bottles. Straight lines fit the equation y=mx+c, and you will be able to see from your graph that the gradient m = 20 and c=0 because the line goes through the origin. This will enable you to read off the graph how many bottles (say) 3.5 cases will contain or calculate it by substuting in the formula now that you have acquired m abd c from the graph.
The "words" you require to answer the question will be of the form "The number of bottles is directly proportional to the number of cases".