This exercise will be more useful if you push yourself to complete all the forms with the correct angles and neater lines. Right now some of your forms are a bit ambiguous, so it'll be hard to apply this into future drawings. I think you should try this again but with more boxes/ cylinders because right now it looks closer to an actual skeleton than a simplified version. It might help to practice drawing some boxes/ cylinders/ spheres by themselves from a few different angles - keeping in mind which lines will be parallel, which lines will converge, how it'll look when foreshortened, etc. I also need to practice this because these forms that I thought were "simple" have more nuance than I thought! Good luck!