A true-false test consists of 10 items. What is the probability that Chris gets 80% or more for the test?

Answer 1

I assume you mean by just guessing.

he must get 8 right, or 9 right or 10 right
= C(10,8)(1/2)^8 (1/2)^2 + C(10,9)(1/2)^9(1/2) + C(10,10) (1/2)^10
= (1/2)^10 ( 45 + 9 + 1)
= 55(1/2)^10
= 55/1024

not good, better study

Answer 2

Hi,
Can you please show the steps for
C(10,8)(1/2)^8 (1/2)^2 + C(10,9)(1/2)^9(1/2) + C(10,10) (1/2)^10
to
= (1/2)^10 ( 45 + 9 + 1)
I am a bit confused.

Thank you in advance :)