Question

So I can get by probability of 4/5 more than 90 percent in any quiz but I don't know how to calculate the probability by which I can get 3 out of 4 tests more 90

Answer 1

Sorry for the usurping of my humor and all

Answer 2

don't worry about it.

lets get back to the question

Answer 3

Yeah

Answer 4

So I thought the answer would be the reasonable 4/5*4/5*4/5

Answer 5

Because I ace the 3 exams out of 4

Answer 6

probability (3 out 4 tests with >90%) = P(1st >90%) x P(2nd >90%) x P(3rd >90%) x P(4th <90%)

Answer 7

P(any test >90%) = 4/5

Answer 8

Looks like a permutation

Answer 9

not quite. its just a multiplication of probabilities because you're looking at getting 3 tests >90% and 1 test <90%, so you just multiply out.
What do you think that the probability of getting <90% will be?

Answer 10

Every time it's 1/5

Answer 11

Oh so 4/5*4/5*4/5*1/5?

Answer 12

yep thats it!!
so whats the final probability?

Answer 13

Like when they say every superman falls from a tree

Answer 14

64/600

Answer 15

with GCF, 32/300=16/150

Answer 16

8/75 yes?

Answer 17

just wait one minute. i'm re-thinking my answer

Answer 18

ok

Answer 19

its not quite right

There are 4 ways that this can happen so we have to multiply the final probability by 4

Answer 20

so we have 4 x (4x4x4)/(5x5x5x5)

Answer 21

256/625

Answer 22

because its 3 out of 4, we could have 4 different ways that this can happen

Answer 23

battery has run out on my notebook so i gotta go

Answer 24

You have tests that are (assumed) independent,
probability of success (\(\ge\)90%) is constant at p=0.8 throughout
You have a number of Bernoulli trials (one of two outcomes)
The number of trials is known (4).
Under all these conditions, you can use the binomial distribution, where the
number of success (s) is given by
\(\large C(n,s) p^s (1-p)^{n-s}\)
and C(n,s) is combination, n choose s, equal to n!/(s!(n-s)!)
and n=4, s=3.
P(3 out of 4) = \(C(4,3)~ 0.8^3 (1-0.8)^{4-3}\)
Take out your calculator and evaluate the probability accordingly!