Question

What data value would be the cutoff for the top 20% from a population with mean = 256 and st.dev = 39

a) 224

b) 295

c) none of these

d) 340

e) 288.8

Answer 1

Given that,

mean = = 256

standard deviation = =39

Using standard normal table,

P(Z > z) = 20%

= 1 - P(Z < z) = 0.20

= P(Z < z ) = 1 - 0.20

= P(Z < z ) = 0.80

= P(Z < z ) = 0.8

z = 0.84 (using standard normal (Z) table )

Using z-score formula  

x = z * +

x= 0.84*39+256

x= 288.8

CORRECT OPTION

e) 288.8