Answer:

n(a n b) = number divisible by both 7 and 11 (i.e. 77)

By using above formula

n(A ∪ B) = n(A) + n(B) - n(A ∩ B)

Number of positive integers below 1000, which are divisible by 7 or 11

n(A ∪ B) = 142 + 90 - 12

⇒ n(A ∪ B) = 220 ) 1000