It is a well-known fact that Dr. Barnes rides a skateboard, sometimes even on campus. Suppose that Dr. Barnes selects a skateboa
rd by first picking one of two skateboard shops at random and selecting a skateboard from that shop at random. The first shop contains two "rad" skateboards and three "gnarly" skateboards, and the second shop contains four "rad" skateboards and one "gnarly" skateboard. What is the probability that Dr. Barnes picked a skateboard from the first shop if he has selected a "gnarly" skateboard?
In total, there are 3 gnarly boards in the first shop and 1 gnarly board in the second. We know that he has selected one gnarly board out of the 3 + 1 = 4 existing boards.
The probability the board came from the first shop is 3 / 4 = 0.75 = 75%.
