How many times the digit 9 occurs in the numbers from 100 to 999?

How many three-digit numbers are divisible by 5?

Seven times a two-digit positive number is equal to four times the number obtained by reversing the order of the digits. How many such two-digit numbers are there?

How many four digit numbers can be formed using the digits 0, 1, 2, 3, 4 without repetition?

How many prime numbers are there between 1 and 50?

What is the digit at the 100th place of number (225)⁴⁰ ?