The ticker timer makes dots on a paper tape every fiftieth of a second. If a piece of tape is pulled through the timer for a second there will be 51 dots on it.
(One of these will be the one at the start!)

There is 1/50 s between dots we must count the number of SPACES and not the number of dots themselves.

To work out the velocity of the tape all we need to do is to measure the length of tape that goes through in the time given by the number of spaces.

If you have a length of tape with 50 spaces then it took 50/50 s or 1s to go through the timer.
So if there are 5 spaces it took 5/50 = 1/10s.