Expert: Sherry Wallin - 5/3/2011

Question
I think I might have known how to do this once.  Right now I can't wrap my head around it, though.  Maybe I'll kick myself when I hear your answer!

John and Barry are courier drivers.
John delivers to the Quick Mart on 3 random days / week, on average.
Barry delivers to the Quick Mart only 1 random day / week, on average.

what is the average amount of days / week that both John and Barry will deliver to Quick Mart on the same day?  

In this case, it's obviously a fraction of a day.  I'm not actually looking for an answer though, more a formula for this scenario.

Thanks

Answer
Mac~

I've not heard of 'random overlaps' so I will approach this just from a probability stand point. Please keep in mind that I may be wrong. When John delivers to Quick Mart there is a 3/7 chance that it will occur on any one day in the week. When Barry delivers to Quick Mart there is a 1/7 chance that he will deliver on any one day. The probability that both of them will deliver on any one day in a week is 3/7 * !/7 = 3/49.

Math Prof