How often Mark paints a 7
Riddle:
How many times does Mark paint the digit 7 on doors 1 to 100?
Answer:
Answer: 20. Explanation: 7 appears once in each of 7,17,27,37,47,57,67,77,87,97 → 10 times. In 70-79, all numbers have a 7 → another 10 times. Total: 20.