Stop, you have to seperate between Pity timer and chance.

The chance may be 1 epic in 5 packs (1 legendary in 20 packs).

But:
But: if you had 4 packs without an epic card, you don’t get an epic in the fifth with 100% (same for legendary and the 20th).

So. That’s about the chances.

Let’s head to the pity timers:
If you had 9 packs without any epic card or 39 packs without any legendary, so you get definitely! An epic card in the 10th and a legendary in the 40th pack.

Short: don’t mix Pity timer and chances