Q5. How many primes are less than 99? Is the set of primes finite?
25 primes are less than 99 (2, 3, 5, 7, …, 89, 97), so that set is finite. The set of all primes is infinite.
Q6. Is {x : x ∈ Z and −10 ≤ x ≤ 10} finite? Find its number of elements.
Finite: the integers from −10 to 10, that is 10 negative, zero and 10 positive, 21 in all.
Q7. Is the set of all lines through a fixed point finite or infinite?
Infinite: there is one line for every direction, and there are infinitely many directions.