Thread: 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.

Please help

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

sd =

Thanks.