# Thread: [SOLVED] Expectation and Variance

1. ## [SOLVED] Expectation and Variance

In a multiple choice test there are 10 queastion and each had 4 choices.

If a student guesses at each of the answer, what is the prob of getting none corrects? /// I just want to prove my answer:

p(X=0)
= 10c0
= 10!/ 0!(10-0)!
= 10!/10!
=1

b.) More than 7 corrects:

10c8 * 10c9 * 10c10

=10!/ 8!(10-8)! * 10!/ 9!(10-9)! * 10!/ 10!(10-10)!
=45 * 10 * 1
=450

*Just wanna know if they are correct!!!tnx

2. I don't think your approach is right. Here is what i would do.

p( getting correct answer ) = 0.25
q = 1 - 0.25 = 0.75
P( of getting none correct ) = 10C0 x (0.25)^0 x (0.75)^10

b.) More than 7 corrects
p( more than 7 corrects ) = P(x=8) + P(x=9) + P(x=10)
= 10C8 x (0.25)^8 x (0.75)^2 + 10C9 x (0.25)^9 x (0.75)^1 + 10C10 x (0.25)^10 x (0.75)^0

Originally Posted by geeko
In a multiple choice test there are 10 queastion and each had 4 choices.

If a student guesses at each of the answer, what is the prob of getting none corrects? /// I just want to prove my answer:

p(X=0)
= 10c0
= 10!/ 0!(10-0)!
= 10!/10!
=1

b.) More than 7 corrects:

10c8 * 10c9 * 10c10

=10!/ 8!(10-8)! * 10!/ 9!(10-9)! * 10!/ 10!(10-10)!
=45 * 10 * 1
=450

*Just wanna know if they are correct!!!tnx