How many Chansey must I catch to guarantee getting a Lucky Egg in Platinum?

Question

its a 5% chance to catch a chansey holding a lucky egg. how many chansey must I catch to make sure one is holding it?

Answer 1

Assuming the statistic you provided is correct, one in every 20 chanseys has a lucky egg. However, you can't catch a certain number to guarentee that you will get one; even if you encounter 20 pokemon it does not guarentee that one of them has a luckey egg, that just is how likley it is that one of them has it. Each encounter has a fresh chance of having a lucky egg; it doesn't become progressivly more likley.

Thus, I would recommend that you use a pokemon with the ability frisk to see if the chansey is in fact holding a lucky egg because there is no real other way to tell and you'll be draining your resources catching that many chanseys in hope of finding one with a lucky egg.

Comments

You could use thief, covet, or trick to get the lucky egg.

Answer 2

As the other commenter noted, you can never guarantee a lucky egg even with 1000 chansey.

However, what I imagine you want to know is how many chansey do you have to catch to receive a lucky egg on average. The answer is 14 chansey. This is because (19/20)^14 is less than 0.5. Therefore, the odds of rolling fourteen 19/20's is less than 50% to happen.

Best

Comments

First, 14 is the median amount of Chanseys you need to encounter to get a lucky egg. The word "average" can refer to the median but usually refers to the arithmetic mean, which is 20 Chanseys in this case.
Second, you can use compound eyes to raise the chance of encountering Pokemon with items. The chance of finding a lucky egg on a Chansey will go up to 7.5%, so there's a 50% chance of getting at least one lucky egg in 9 Chanseys.