a)
X ~ N ( µ = 2800 , σ = 450 )
We covert this to standard normal as
P ( X < x) = P ( (Z < X - µ ) / σ )
P ( X > 3100 ) = P(Z > (3100 - 2800 ) / 450 )
= P ( Z > 0.67 )
= 1 - P ( Z < 0.67 )
= 1 - 0.7486
= 0.2514
= 25.14%
b)
P ( 3100 < X < 3500 ) = P ( Z < ( 3500 - 2800 ) / 450 )
- P ( Z < ( 3100 - 2800 ) / 450 )
= P ( Z < 1.56) - P ( Z < 0.67 )
= 0.9406 - 0.7486
= 0.1920
= 19.20%
c)
P ( 2250 < X < 3500 ) = P ( Z < ( 3500 - 2800 ) / 450 )
- P ( Z < ( 2250 - 2800 ) / 450 )
= P ( Z < 1.56) - P ( Z < -1.22 )
= 0.9406 - 0.1112
= 0.8294
= 82.94%
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