Counting numbers in other lines

[perfringo]

This is homework so it's about learning.

There is requirement to use str.startswith(). In order to get help fire up interactive interpreter and:

>>> help(str.startswith)
Help on method_descriptor:

startswith(...)
    S.startswith(prefix[, start[, end]]) -> bool

    Return True if S starts with the specified prefix, False otherwise.
    With optional start, test S beginning at that position.
    With optional end, stop comparing S at that position.
    prefix can also be a tuple of strings to try.

Now you know what startswith does. Pending on other requirements you have not disclosed there are several avenues to proceed. You can start at specific index (if content before : is always the same); you can split, strip and provide tuple of numbers to check whether reminder starts with number etc, etc.