COWA19E-Editorial

PROBLEM LINK:

Practice
Contest

World War Preparation

Lets remove the chef bit aside and map the problem into little mathematical scenario.
We have an event happening n times with only two possible outcomes (sucess or failure),
with following assumption made in the problem:
(a) These events are independent and identically distributed (i.i.d)
(From the fact that skill level remains constant)
(b) these events has the probabililty (say p), which is equally likely between 0 and 1.
(From the fact that the probability is uniformly distributed)

Claim: the probability of n+1 event will be sucess given in n event r of them were sucess
is given by (r+1)/(n+2).

Hardcore Mathematical Proof:
Lets denote each of these events to be Bernoulli trails. Lets say X_i be 0 or 1 to denote
the ith trial for sucess or failure.
Lets say

S_n = X_1 + X_2 + ... + X_n

So
According to problem we need to find P(X_n+1=1 | S_n = r).
This was just to give you insight and total proof can be found here(https://jonathanweisberg.org/post/inductive-logic-2/).

Another loose but Smart Proof:
Suppose we have a marble between two wooden planks. we roll it between them and it stopped
due to friction.
Now we roll total n(say 5 for illustration) marbles, r(say 3) of them went on left side of
first ball and rest(2 in this case) right of first ball.
So now if we roll last ball the cases can be

# L L L ** R R
L # L L ** R R
L L # L ** R R
L L L # ** R R 
L L L ** # R R
L L L ** R # R
L L L ** R R #

# represent the last ball ** represent the first ball. L and R are the left and right balls
respectively 

Now we can easily say that probability of last ball coming out in left is 4/7 as all left
to right arrangements are equally likely a priori.

This whole scenario can be easily mapped with Bernoulli trial of sucess and failure to the
ball landing on left or right, all balls are independent are also taken care.
So in general case probability will be (r+1)/(n+2).