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A report I just made up says that 11% of all flowers sold from a particular flower shop are carnations. If this flower shop sells 316 bouquets on a random day, what is the probability that:

a) fewer than 40 are carnations?

b) at least 25 are carnations?

c) between 25 and 40 are carnations?

NOTE: A THROUGH C ARE NOT ANSWERS. THEY ARE PARTS OF THE QUESTION THAT NEED TO BE SOLVED INDIVIDUALLY.

1 Answer

7 votes
This is a problem on a binomial probability distribution. A binomial probability distribution has fixed number of trials, n. In this case, n is equal to 316. Also, the trials in a binomial probability distribution must be independent. Each trial must have all outcomes classified into two categories--carnation or not carnation. Lastly, the probability of success remains the same in all trials.

The binomial probability formula is given by

p\left(x\right)=C\left(n,\:x\right)p^x\left(q\right)^(n-x)
where
n=number of trials
x=number of successes among n trials
p=probability of success in any one trial
q=probability of failure in any one trial (q=1-p)

So, from the given problem, the following quantities are given:

n=316,

p=0.11

q=1-0.11=0.89

Part A
The probability of obtaining fewer than 40 (0 to 39) carnations is the summation of all the binomial probabilities from 0 to 39. That is

p\left(x\ \textless \ 40\right)=\sum _(x=0)^(39)\:C\left(316,\:x\right)\left(0.11\right)^x\left(0.89\right)^(316-x)=0.8048

Part B
The probability that at least 25 are carnations is the sum of binomial probabilities of x from 25 to 316. That is

p\left(x\ge 25\right)=\sum _(x=25)^(316)\:C\left(316,\:x\right)\left(0.11\right)^x\left(0.89\right)^(316-x)=0.9720

Part C
The probability that carnations are between 25 and 40 is given by the sum of individual binomial probabilities from 26 and 39. That is

p\left(26\le x\le 39\right)=\sum _(x=26)^(39)\:C\left(316,\:x\right)\left(0.11\right)^x\left(0.89\right)^(316-x)=0.7615

Therefore, the probability that there are fewer than 40 carnations is 0.8048, at least 25 carnations is 0.9720, and between 25 and 40 is 0.7615.
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