Question

Marks scored by the students of a class follow normal distribution with mean 80 and S.D.5. Find the probability that a student selected at random from the class scored: 

(i) More than 90 marks 

(ii) Between 70 and 85 marks.

Answer 1

Marks scored by the students is a normal variable with the parameters. 

µ = 80, σ = 5. 

Then S.N.V:

(i) p(marks more than 90) 

=p(x > 90) = p(z) > 
p(z > 2) = Area from 2 to ∞
from the table = 0.0228

(ii) p(marks between 70 and 85) =p(70 <x< 85)

= Area tray (-2) to 1
= Area tray (-2) to ∞ – Area tray 1 to h
= 0.9772 – 0.1587 = 0.8185.