Python beginner

[buran]

If you add some comments it should be clear for all.

    for month in range(months):         # count once for each month
        borrowed *= 1 + interest / 12   # & compound the interest
        borrowed -= repayment           # & subtract repayment
    print(borrowed)                     # then, print the remaining amount borrowed

Sorry, but I would disagree. What you provide is really bad example of comments (or if you want - really good example of bad comments). Check PEP8 about inline comments
Overuse of inline comments makes it hard to read the code and for simple lines (like these) are completely redundant. Good descriptive variable names would do. A short single-line block comment may be OK, but inline comments - definitely not. And given that it is actually a function - a docstring would be more than enough in this case (i.e. for a simple function like this one)...