In a certain town there are 2 bus companies whose buses stop at the Main Street station. One company's busses run every 8 minutes, the other runs every 10 minutes - but the times of arrival of the previous busses are unknown.

Question

a) In a certain town there are 2 bus companies whose buses stop at the Main Street station. One company's busses run every 8 minutes, the other runs every 10 minutes - but the times of arrival of the previous busses are unknown. The question is, what is the average length of time that you will wait for a bus after at the station?
b). Same as the above, but 3 bus companies - - whose busses each stop every 10 minutes.