In: Statistics and Probability
Private nonprofit four-year colleges charge, on average, $26,470
per year in tuition and fees. The standard deviation is $6,683.
Assume the distribution is normal. Let X be the cost for a randomly
selected college. Round all answers to 4 decimal places where
possible.
a. What is the distribution of X? X ~ N(,)
b. Find the probability that a randomly selected Private nonprofit
four-year college will cost less than 30,818 per year.
c. Find the 74th percentile for this distribution. $ (Round to the
nearest dollar.)
average is $26,470 and standard deviation is $6,683.
a) the distribution of X ~ N(26470, (6683)2)
b) we have to find P (X ≤ 30818).
μ=26470, σ=6683
P (X ≤ 30818) = P ((X- )/ ≤ (30818- 26470)/6683)
= P( Z ≤ 0.6506)
= 0.7423
Hence P (X ≤ 30818) = 0.7423
c) 74th percentile for this distribution
First, we need to find the z-score associated to this percentile.
P (Z< Zp) = 0.74
The value of Zp that solves the equation above cannot be made directly, it is solved either by looking at a standard normal distribution table or by approximation.
Based on this, we find that that the solution is Zp=0.643, because from the normal table we see that
P(Z< 0.643) = 0.74
Therefore, the percentile we are looking for is computed using the following formula:
P74 = μ + Zp × σ
P74 = 26470 + 0.643 ×6683
= 30769.477
Therefore, it is concluded that the corresponding 74th percentile is found to be P74 = 30769.477.
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