Please help me understand this

Orders arrive one-by-one according to a Poisson process with an average rate of 36 per hour. Independent of everything else, each order is for product A, B or C with respective probabilities 0.2, 0.3 or 0.5 (note they add to 1). Answer the following independent questions:1. What is the mean and standard deviation of the total number of orders that arrive in a four- hour period