Thirteen problems from Codeforces and AtCoder, the two biggest competitive programming judges. They are submitted on those sites. Each one is easiest with a helper function or two: a digit sum, a palindrome check, a digit count, a prime test, a divisor count. They go roughly from the first lesson's single functions to problems you split into several.
The statements are in English (and Japanese, on AtCoder). Before you code, say
which helpers you need — and read the limits: Digits in Multiplication needs
long long and the √N idea from the decomposition lesson, Nearly Lucky
Number reads a number with up to 18 digits, and Odd Divisor numbers up to 10¹⁴.