How many numbers between 50 to 100 are divisible by 3?
Just divide:
100 / 3 - 50 / 3 =17. Round to the next whole integer. That is how many numbers are divisible by 3.
51, 54, 57, 60, 63, 66, 69, 72, 75, 78, 81, 84, 87, 90, 93, 96, 99.