The resultant velocity of the plane is the sum of the two velocity vectors which are perpendicular to each other. See the attached figure.
The magnitude of the resultant velocity is
.
The approximate value of the actual velocity of the plane is . Correct choice is (D).
Assume a binomial distribution. Then the mean of the distribution is , where
is the number of people taken in a sample and
is the probability of the mutation occurring in any given person, and the standard deviation is
. Then in this case, the standard deviation is
.