Loop until a number is entered that matches the criteria.

cpsc120_while_loop_prompt.cc

// Gist https://gist.github.com/450b2211f29cc7cae4735833f96bdedb

#include <iostream>

int main(int argc, char const *argv[]) {
  std::cout << "Enter a number between 10 and 20: ";
  int input_number{0};
  std::cin >> input_number;
  while (input_number < 10 || input_number > 20) {
    std::cout << "Enter a number between 10 and 20: ";
    std::cin >> input_number;
  }
  std::cout << "Thank you!\n";
  return 0;
}