Okay, I think I figured out the problem. The math definitely doesn't make sense because I just looked at the label for the package of the jelly rolls and it is a 20 piece set, not the traditional 38 piece set. So... DOH on me! Knowing that, SewCraftyGirl, your response seems to be correct.
Thank you for all the reading and responses! I guess I better get my hands on another compatible jelly roll. I bought the last one at JoAnn's, and it was the last one of its colors. Time to shop online!