# Math Help - standard deviation

1. ## standard deviation

There are 3000 cars travelling between St.Paul and McGregory each day. 38% is the probability that a motorist is speeding on this highway. Calculate the standard deviation for the number of cars

sd = sqr(nxpx1-p)

I know the answer is 26.59 but I don't get this answer.

2. n= the number of cars = 3000, p = the probability of a car speeding.

sd = $\displaystyle \sqrt{np(1-p)}= \sqrt{3000\times .38\times (1-.38)} = 26.59$

3. Thanks.