How likely is it to get a repair book with the fishing rod in Minecraft?
0.028% with Luck of the Sea 3
0.012% without Luck Enchantment
So extremely unlikely. You have to fish an average of 3571 times (with Luck3). But in connection with an AFK fish farm, that's clear.
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Calculation method and sources:
The probability that a book is a mending book is 2/136. That's 1.4706%.
(https://minecraft.gamepedia.com/Enchanting#Summary_of_enchantments)
The probability to fish a book is:
With Luck of the Sea 3: 1/6 * 11.3% = 1.8833%
Without Luck of the Sea: 1/6 * 5% = 0.8333%
(https://minecraft.gamepedia.com/Fishing#Junk_and_treasure)
0.01470588 * 0.018833 = 0.00027696 = 0.028%.
0.01470588 * 0.008333 = 0.00012254 = 0.012%.
