You can count by fives by adding five to the previous count.

If you start with a zero then each number will end in either 5 or 0.

The numbers that you would count if you started with 0 and counted by fives would be:

0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85,
90, 95, 100.

If you start the count with another number, then just add five to the last count.
For example:

1, 6, 11, 16, 21, 26, 31, 36, 41, 46, 51, 56 and so on.

Notice that there is a regular pattern in these counts.
Every other number ends with the same digit.