Challenge 1: Flexible Greeting — Possible Solution
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def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

print(greet("Ada"))                             # just a name
print(greet("Ada", "Hi"))                       # both, positional
print(greet(greeting="Hey", name="Ada"))        # both, keyword, reversed order

Output:
Hello, Ada!
Hi, Ada!
Hey, Ada!

WHY THIS WORKS AS AN ANSWER
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greet("Ada") supplies only the required positional parameter name;
greeting is left unset, so it falls back to the default value "Hello"
defined in the function signature, exactly like this chapter's
describe_pet(name) example that relied on animal's default.

greet("Ada", "Hi") supplies both arguments positionally, in the exact
order they appear in the function signature (name, then greeting) —
"Ada" fills name, "Hi" fills greeting.

greet(greeting="Hey", name="Ada") supplies both arguments as keyword
arguments, and — as the chapter states — keyword arguments can be
passed in ANY order at the call site, since each value is explicitly
tied to a parameter name rather than a position. greeting="Hey" is
written first here even though greeting comes second in the function
definition, and the call still works correctly.