ASAP please!!!

A computer store has only three laptop computers in stock currently. The manager examines historical sales data and finds the number of laptop computers sold daily has the distribution defined below, where S is the random variable representing the number of computers sold for any given day.

s 0 1 2 3 4 5 fS(s) .15 .35 .25 .15 .05 ? A. To be a valid probability mass function, what does fS(5) = P(5) need to be? B. What is Fs(s)? C. What is the probability the store will have enough laptops for all the customers who want to buy one today? D. What is the expected number (mean) of laptops the store sells for any randomly selected day? E. What is the standard deviation of the number of laptops the store sells for any randomly selected day?