The fourth proportional to 5, 8, 15 is  Choose the correct option:
A) 18  B) 19  C) 20  D) 24  E) None
Sol.   Let the fourth proportional to 5, 8, 15 be x.   Then, 5 : 8 : : 15 : x ⇔ 5x = (8x15)   ⇔ x = (8x15) / 5 = 24.   Answer 24
<result>D</result>
The fourth proportional to 5, 8, 15 is