Which of the follo9wing numbers will replace the question mark (?) in the given series?
5, 6, 10, 12, 15, ?, 20, 24

5, 6, 10, 12, 15, ?, 20, 24

Step I, Step III, Step V (Table of '5') = 5 , 10 , 15, 20 

Step II, Step IV , Step VI (Table of '6')= 6, 12 , 18 , 24

Thus the sixth step is $$6 \times3$$ = 18

Thus,  the correct answer is (C).