For those, like me, who always want to find a Pikachu before leaving Viridian forest.
This explains a lot
The probability of finding a Pikachu in Viridian Forest is 5% so you just need 20 encounters to ensure finding a Pikachu, right? Math doesn't work out like that. Let's do the math to find the probability of encountering at least one Pikachu in 20 encounters.