How many numbers between 1 and 100 are divisible by 9 and 10?

Answer 1

There are 11 divisible by 9 (which are 9, 18, 27, 36, 45, 54, 63, 72, 81, 90, 99);

There are 9 (or 10) divisible by 10 (which are 10, 20, 30, 40, 50, 60, 70, 80, 90; and 100 if it is 1 to 100 inclusive);

There is 1 divisible by both 9 and 10 (which is 90).