" A contractor has recently purchased a new bulldozer. On previous jobs, 15 out of a total of 50 bulldozers have broken down before the end of the job. What is the mean and standard deviation of the probability distribution describing the probability of failure of a bulldozer? Note: assume a normal approximation is applicable." Mean of the probablity distribution seems obvious (0.3), but how does one find the standard deviation?? Thanks for any help that can be offered.