Starting with the cylinders around arbitrary minor axes, these are largely very well done. You've been quite thorough in checking your ellipses' alignments and have identified even minor discrepancies. This is excellent, as it is precisely how we continue to improve, even as our results become strong enough to fool the naked eye. Your ellipses are also confidently drawn, maintaining proper evenness of shape, and remaining pretty tight whilst drawing through them.

I have just one minor point to call out, although there are two parts to it. Firstly, when doing a challenge that requires a lot of repetition, always try and work in a good deal of variation. Specifically in this case, vary the rate of foreshortening across your cylinders, drawing some with shallower foreshortening and others with much more dramatic foreshortening.

Not only is this good for just getting more varied practice, but it also helps highlight potential misunderstandings that I can identify. In this case, there is an issue I wanted to call out, although it's only slightly present in your work. Still, it's worth mentioning.

When drawing our cylinders, we need to consider the differences between the ellipses on either end. We know that they're going to change in two ways - first of all, the far end is going to be smaller in overall scale than the closer end, as we understand from the basic principles of perspective. Secondly, the far end is also going to be wider in degree than the closer end. Both of these, however, happen together. As we get more foreshortening applied to the form, we're going to see more of both of these changes. Never will we see lots of one, and less of the other.

Now, I'd have been able to call this out more firmly if you'd pushed the foreshortening of your cylinders more, but there are some cases - like cylinder 110 for example, where the overall scale of the far end doesn't get smaller at all, but the degree does get wider. This leads to a contradiction, which makes the cylinder feel off to us. In general, you should never be drawing anything that has no foreshortening at all, so at the very least the farther end should be at least a little smaller, and the degree shift should match.

Moving onto your cylinders in boxes, while you were definitely applying the principles of the exercise properly, I think you may not have necessarily been thinking too much about what the line extensions were telling you about where your mistakes were, and how to improve upon them - at least not for the first while. This improved in the second half of this section.

This exercise is basically about learning how to estimate boxes that are roughly square in proportion on either end. We achieve this similarly to how the box challenge teaches us to get our convergences to be more consistent towards shared vanishing points - through the line extensions.

In this exercise, we add three more lines per ellipse to the extensions. These lines will align towards the box's vanishing points only when the ellipse itself represents a circle in 3D space that sits on the surface of the box. Therefore making adjustments to bring those lines more in line with the box's vanishing points will help us improve the proportions of our box.

In your case, through the first half you were still ending up with a lot of wild proportions. By the end however, you did make a marked improvement, and while there is still lots of room for improvement, it's heading in the right direction. Just be sure to keep up with this exercise, and remember that it's about the boxes rather than the cylinders.

I'll go ahead and mark this challenge as complete.