The base case is when (single digit).
The sum is just itself. Recursive formula: python def sum_of_digits(n): if n < 10: return n return (n % 10) + sum_of_digits(n // 10) Tracing sum_of_digits(1234): plaintext sum_of_digits(1234) = 4 + sum_of_digits(123) sum_of_digits(123) = 3 + sum_of_digits(12) sum_of_digits(12) = 2 + sum_of_digits(1) sum_of_digits(1) = 1 ← Base case! Working back up: Each call peels off the last digit and recurses on the rest.