Value of za/2 for 80% Confidence Interval: .255

[Agent_J]

What is the value of za/2 for an 80% confidence interval?
a. .255.
b. 1.28.
c. .0398.
d. .525.

i know the answer is a, but does anyone know how to calculate that ?

 

[HallsofIvy]

What is the value of za/2 for an 80% confidence interval?
a. .255.
b. 1.28.
c. .0398.
d. .525.

i know the answer is a, but does anyone know how to calculate that ?

Not in any easy manner- you look it up in a table!
Here's one on the internet:
http://people.hofstra.edu/faculty/Stefan_Waner/RealWorld/normaltable.html