21. Let P1 be the set of all primes, 2, 3, 5, 7, . . . , and for each integer n, let Pn be the set of all prime multiples
of n, 2n, 3n, 5n, 7n, . . . . Which of the following intersections is nonempty?
(A) P1 P23 (B) P7 P21 (C) P12 P20 (D) P20 P24 (E) P5 P25
Is there a number-theory method to do this question?
My method was to note that P_12 and P_20 both contain 60=5*12=3*20.
Thank you very much.