Imagine you are organizing a party and want to give party favors to guests who have brought both sweets and drinks. You have a total of z guests, and n guests have brought sweets, and m guests have brought drinks. You want to know how many guests brought both sweets and drinks, so you ask your friend to count the number of guests x such that x<=z and x%m ==0 and x%n==0. Your friend comes back with the answer - the number of guests who brought both sweets and drinks is equal to the number of integers x such that x<=z and x%m ==0 and x%n==0.