Consider three telephone circuits from the USA to Europe belonging to a company. Measurements reveal that during the business day each circuit is busy 10 min out of an hour, on average. Most calls are short (3 or 5min or less).
(a) Let p be the independent probability that a single circuit is busy. Find a numerical value of p and show how you arrived at it.
(b) Write an expression for the probability a given call goes through (finds a free circuit). 
(c) Write an expression for the probability that, at a given instant, exactly one circuit is free. 
(d) Write an expression for the average number of busy channels. Hint: Write it as a weighted sum.