12.13 - rewrite the 12.13 solution from PS3 (yours or mine, your choice) to use let to simplify and clarify the code.
Note: if you're having trouble figuring out how to come up with the right base cases and the right recursion relations, an excellent study technique is to try a bunch of non-assigned problems, and then ask me or the TAs to help you solve any you have trouble with. This is a skill like skiing, and watching other people ski can get you only so far - you have to get your recursion legs through practice.
All the files should be legal scheme code, with comments indicating which fragment of code corresponds to which problem, and comments with any answers to questions that require answers to be in English rather than Scheme.You must submit all your files at once! You are allowed to make corrections, because your last submit command overwrites any data previously submitted.