Two teachers Xavier and Young use the computer in the principal’s office to generate work for their mathematics class. Every afternoon both teachers go to the principal’s office between 4:00 and 5:00 pm to run their programs. Each program requires 10 minutes of computer time. No one else at the school ever uses the computer during this time, so Xavier and Young usually do not have to wait to get on the computer. But sometimes one of them is already on the computer when the other arrives to use it. Assuming both teachers arrive at random times between 4:00 and 5:00, what is the probability that one of them will have to wait to get on the computer?