01 If K = {x : x ∈ N, x < 40 and x has exactly three positive divisors}, what is K?
Answer and explanation
Correct answer: A. K = {4, 9, 25}
Explanation: A positive integer has exactly three positive divisors precisely when it is the square of a prime: if x = p², its divisors are 1, p, and p². The prime squares less than 40 are 2² = 4, 3² = 9, and 5² = 25. The next prime square, 7² = 49, is not less than 40. Hence K = {4, 9, 25}.