We can solve this problem by using the formula for the number of elements in the union of two sets:
n(A U B) = n(A) + n(B) - n(A ∩ B)
where n(A) represents the number of elements in set A, n(B) represents the number of elements in set B, and n(A ∩ B) represents the number of elements in the intersection of sets A and B.
In this case, we have:
n(A) = 250
n(B) = 250
n(A U B) = 500 - 20 = 480
Substituting these values into the formula, we get:
480 = 250 + 250 - n(A ∩ B)
Solving for n(A ∩ B), we get:
n(A ∩ B) = 250 + 250 - 480 = 20
So, the number of customers who regularly buy both products A and B is 20.
To find the number of customers who regularly buy only product A, we can subtract the number of customers who regularly buy both products A and B from the total number of customers who regularly buy product A:
n(A) - n(A ∩ B) = 250 - 20 = 230
Therefore, the number of customers who regularly buy only product A is 230.