To find the probability of randomly selecting an employee who eats breakfast or lunch at the office, we need to consider the employees who eat breakfast, lunch, or both.

Number of employees who eat breakfast = 20
Number of employees who eat lunch = 50
Number of employees who eat both breakfast and lunch = 15

Total number of employees who eat breakfast, lunch, or both = 20 + 50 - 15 = 55

Therefore, the probability of randomly selecting an employee who eats breakfast or lunch at the office is 55/80, which can be simplified to 11/16.

So the probability is 11/16.