Consider a station consisting of three machines in parallel (i.e. that perform the same operation) and that have different processing speeds. Machine 1 processes an average of one part every 4 minutes. Machine 2 processes an average of one part every 5 minutes. Machine 3 processes an average of one part every 2 minutes. Each part requires processing at only one machine before it leaves the station. Assuming the machines work non-stop and are never starved of parts, what is the average number of minutes between part completions for the station as a whole?