How do you solve "z+ma=ba" solve for a

On saturday morning, calls arrive at ticketmaster at a rate of 108 calls per hour. what is the probability of fewer than three calls in a randomly chosen minute?

International calls require the use of a country code. many country codes a 3-digit numbers. country codes do not begin with a 0 or 1. there are no restrictions on the second and third digits. how many different 3-digit country codes are possible?

If 10 men take 18 days to mow 60 acres of grass, how long will 15 men take to mow 80 acres?