# Math troubles

There's 5 multiple choice questions on a quiz. Four choices to each question. Find the probability that the student gets 3 or more questions right.

p(x=3) (.25)^3= 0.015
p(x=4) (.25)^4=3.906
p(x=5) (.25)^5=9.765

Do you add them all together to get the overall probability?

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1. First off, no probability can be more than 1, so you calculations are off.

P(x=3) = 5C3 .25^3 .75^2
P(x=4) = 5C4 .25^4 .75^1
P(x=5) = 5C5 .25^5 .75^0

Having gotten those, yes, you add them up.

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2. I calculated them on my calculator so I borrowed my friends (since mine was messing up).

I got
P(x=3) = 5C3 .25^3 .75^2 = 0.008
P(x=4) = 5C4 .25^4 .75^1 = 0.003
P(x=5) = 5C5 .25^5 .75^0 = 0.0007

Did I calculate correctly

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3. Hmmm. I got .08789 + .01465 + .00097 = .10352

Looks like you have some problems. Make sure you are calculating your binomial coefficients correctly.

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4. Where did you get the .25 and .75 from?

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