Answer:
96.42% probability that the number of tails is between 40 and 60.
Step-by-step explanation:
I am going to use the binomial approximation to the normal to solve this question.
Binomial probability distribution
Probability of exactly x sucesses on n repeated trials, with p probability.
Can be approximated to a normal distribution, using the expected value and the standard deviation.
The expected value of the binomial distribution is:
The standard deviation of the binomial distribution is:
Normal probability distribution
Problems of normally distributed samples can be solved using the z-score formula.
In a set with mean 
and standard deviation 
, the zscore of a measure X is given by:
The Z-score measures how many standard deviations the measure is from the mean. After finding the Z-score, we look at the z-score table and find the p-value associated with this z-score. This p-value is the probability that the value of the measure is smaller than X, that is, the percentile of X. Subtracting 1 by the pvalue, we get the probability that the value of the measure is greater than X.
When we are approximating a binomial distribution to a normal one, we have that 
, 
.
In this problem, we have that:
100 tosses, so 
Two outcomes, both equally as likely. So 
So
Estimate the approximate probability that the number of tails is between 40 and 60.
Using continuity correction.
This is the pvalue of Z when X = 60.5 subtracted by the pvalue of Z when X = 39.5. So
X = 60.5
has a pvalue of 0.9821
X = 39.5
has a pvalue of 0.0179
0.9821 - 0.0179 = 0.9642
96.42% probability that the number of tails is between 40 and 60.
