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Sagot :
Step-by-step explanation:
Understanding the Problem
We’re dealing with a normal distribution of test scores. We know the average score (mean) and how spread out the scores are (standard deviation). We want to find a specific test score © where 81% of the students scored lower than that score.
Using the Z-Score
Convert to a Z-score: We need to convert our desired probability (81%) into a standard z-score. A z-score tells us how many standard deviations away from the mean a particular value is. You can use a Z-table or a calculator to find the z-score that corresponds to a cumulative probability of 0.81 (representing 81%). You’ll find that the z-score is approximately 0.88.
Apply the Z-score formula: The z-score formula is:
z = (X - μ) / σ
Where:
z = z-score
X = the raw score we’re trying to find (in this case, C)
μ = the mean (74)
σ = the standard deviation (11)
Solve for C: Plug in the values and solve for C:
0.88 = (C - 74) / 11
Multiply both sides by 11: 9.68 = C - 74
Add 74 to both sides: C ≈ 83.7
Answer
The test score C, rounded to one decimal place, is 83.7. This means that approximately 81% of the students scored lower than 83.7.
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