question on normal distribution

if the mean mark is 75 with a sd of 8 marks, what should you score to be in the top 25% .


z = (x - 75) / 8 is the standard normal variate

we have to find k so that

P[X > k ] = 0.25

P[ z > (k-75)/8 ] = 0.25


refer standard normal tables (note that probability is given)

(k-75)/8 = 0.675 (approximately)

k = 80.4 marks approximately


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