An item has a 5% drop rate. I need 4 of them. How many monsters

Any given fight gives you a 1/20 chance to get the item. So, after about 80 fights, you have a decent chance of having 4.
However, item drop probability doesn't work on this perfect line; imagine trying to guess what number a rolling 20-sided die will land on, and that's about what you've got for your item drop chance.
Are you farming for Ribbons?

chance of not having 4 <5% ???

Not having 1 on 1 monster = 95%
Not having 1 on 2 monsters = 95^2%

Not having 1 on 59 monsters < 5%
repeat x4 = 236 monsters

2360 Minutes = 40 hours farming.

Have fun!

>>145750
>236
shit.

Well, that's what I need to know. I'm going to have a hell of a week. This event ends next Wednesday.