# Thread: standard deviation and probility

1. ## standard deviation and probility

hi everyone i really need help with a question or a few i am really lost in this class so any help would be great

Q- 200 randomly stopped cars, drivers asked if any previous crashes. passed events have proven that 12% of drivers have had previous accidents.

I was to work out the expected value of passed crashes and got 0.12x200=24
and the standard deviation which i got 4.595650117

i am now being asked what the probability that the number of passed drivers having accident before will be exactly the equal to the expected value

if you could explain how you got the answer to please thanks

2. ## Re: standard deviation and probility

it looks like you want

$P(X=24)$

which is
${200 \choose 24}p^{24} (1-p)^{200-24}$

where p would be .12