In: Math
You have 120 mice lacking insulin receptors in their brain tissue. On average 15.2% of these types of mice will die within a month. What is the probability at least 100 mice live until next month? (use binomial approximation of normal)
Solution:
Given,
n = 120
On average 15.2% of these types of mice will die within a month.
 On average
84.8% of these types of mice will live until the next month.
Let X denotes No. of mice out of 120 that will live until the next month.
 p = 84.8% =
0.848
 X follows
Binomial(120 , 0.848)
According to the Normal approximation to the binomial , X follows Normal distribution with
Mean = 
 = n * p = 120 * 0.848 = 101.76
Standard deviation = 
 =
n*p*(1-p)
= 
120*0.848*0.15.2 = 
15.46752 = 3.93287681
Find P(at least 100 mice live until next month)
= P(X  
 100)
= P(X > 99.5) .... Continuity correction
=  P[(X - 
)/
>  (99.5 - 
)/
]
= P[Z > (99.5 - 101.76)/ 3.93287681]
= P[Z > -0.5746]
= 1 - P[Z < -0.5746]
= 1 - 0.2828 ( use z table)
= 0.7172