Bluetick1010

It is a cumulative density function.

That's actually not something I understand either.

It says:

"For the probability density function, Prob (x) = x/15 for x = 1, 2, 3, 4, 5 the CDF is:

[Sigma] x/15, so that F(3) or Prob (x < or equal to 3) is 1/15 + 2/15 + 3/15 = 1/3.

This simply is the sum of the probabilities of 1, 2, and 3. Note that
Prob (x = 3, 4) can be calculated as F(4) - F(2) = 10/15 - 3/15 = 7/15.

I don't understand:

1) How 1/15 + 2/15 + 3/15 = 1/3 (it would just need to be 2/15 + 3/15)

2) Why he subtracts by F(2)