Challenge 1: Sum of Even Numbers — Possible Solution
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total = 0

for n in range(1, 21):
    if n % 2 == 0:
        total += n

print(total)

Output:
110

WHY THIS WORKS AS AN ANSWER
------------------------------
range(1, 21) produces 1 through 20 inclusive — the chapter's own
range() section explains that the stop value is exclusive, so reaching
20 requires stopping at 21, not 20.

n % 2 == 0 reuses the modulo divisibility check from the previous
chapter (py1-3) to identify even numbers — a number is even exactly
when the remainder of dividing by 2 is 0.

total is initialized to 0 before the loop and accumulated with
total += n on each matching iteration, the standard loop-accumulation
pattern: a running variable declared outside the loop, updated inside
it, and read only after the loop finishes.