Posted by Anonymous on .
Statistics show that about 5% of all males are colorblind. Suppose that 20 males are selected at random.
What is the probability that at least 4 of the 20 people are colorblind?
My answer: I got 10.984
The answer key says 0.00257394

Probability 
Reiny,
at least 4 means
4 or 5 or 6 or ... or 20 are colourblind
so exclude cases of none, 1 , 2, and 3
colourblind = .05
not colourblind = .95
None:
C(20,0) (.05)^0 (.95)^20 = .358486
one:
C(20,1) (.05) (.95)^19 = .37735..
two:
C(20,2) (.05)^2 (.95)^18 = .1886768
three:
C(20,3)(.05)^3 (.95)^17 = .059582..
adding up those 4 and subtracting from 1 I got .0159
which is the same result as you had
mmmhhhh?