# User:Jzhang/Segment2

Solutions for segment 2.

## Skilled problem

### Problem 1: If the knight had captured a Gnome instead of a Troll, what would his chances be of crossing safely?

Let

$H_1$ be the hypothesis that he is on bridge without troll
$H_2$ be that he is on bridge with one troll
$H_3$ be that he is on bridge with two trolls

The probability of crossing safely(or on the bridge without troll) is:

$P(H_1|G) = \frac{P(G|H_1)*P(H_1)}{P(G)} = \frac{P(G|H_1)*P(H_1)}{P(GH_1)+P(GH_2)+P(GH_3)} = \frac{1*\frac{3}{5}}{1*\frac{3}{5}+\frac{4}{5}*\frac{1}{5}+\frac{3}{5}*\frac{1}{5}} = \frac{15}{22}$

### Problem 2: Suppose that we have two identical boxes, A and B. A contains 5 red balls and 3 blue balls. B contains 2 red balls and 4 blue balls. A box is selected at random and exactly one ball is drawn from the box. What is the probability that it is blue? If it is blue, what is the probability that it came from box B?

The probability of be blue ball is: $P(b) = P(bA)+P(bB) = \frac{1}{2}*\frac{3}{8}+\frac{1}{2}*\frac{4}{6} = \frac{25}{48}$

The probability of being box B given blue ball is: $P(B|b) = \frac{P(b|B)*P(B)}{P(b)} = \frac{16}{25}$

## Thought problem

All the work is done in team with Sean Trettel

### Activity 1

• Simulate the Knight/Troll/Gnome problem 100,000 times.
• Plot (fraction of safe crossings so far) vs. (number of simulated trials so far) to confirm that this fraction converges to the probability calculated in the segment.

The idea is simulating the whole world with a list of bridges, each bridge is represented with a list, there're five species on each bridge, which are represented with 5 numbers in the list, 1 means Gnome, -1 means Troll.

In each run of the simulation, a random integer number between 0~4 will determine which bridge the knight is on, and another random integer number between 0~4 will determine which specie the knight capture. If the knight capture a Troll, the number of capturing Troll is incremented, and whether he can cross safely is calculated (If the rest four species are Gnomes, it's safe) by summing rest of the list, if it's equal to 4, there're 4 Gnomes left, it's safe, and number that he can cross safely is incremented.

Here's the python code:

import pylab
import numpy
import random
b1 = [1,1,1,1,1]
b2 = [-1,1,1,1,1]
b3 = [-1,-1,1,1,1]

world = [b1,b1,b1,b2,b3]

cTroll = 0
cSafe = 0
frac_list = []

for i in range (10**5):
# choose the world
bc = world[random.randint(0,4)]
# capture one random specie
capture = bc[random.randint(0,4)]
if capture < 0: # capture the troll
cTroll+=1
if sum(bc)-capture == 4: # cross safely
cSafe+=1

# record fraction at every step
if not cTroll:
frac_list.append(0)
else:
frac_list.append(cSafe*1.0/cTroll)

# plot the fraction verse the simulation time
x = numpy.arange(0,10**5,1)

pylab.plot(x,frac_list)
pylab.xlabel('simulation times')
pylab.ylabel('fraction')
pylab.grid(True)
pylab.show()

print cSafe
print cTroll
print cSafe*1.0/cTroll


The calculated fraction is 0.329046526868. The plot is shown here: