Find the fourth proportional to 3, 7 and 15.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 35
The fourth proportional to \(a\), \(b\) and \(c\) is the number \(d\) for which \(a:b=c:d\); the product of the means equals the product of the extremes, so \(ad=bc\).
By the definition, \(3:7=15:d\)
Product of the extremes = product of the means: \(3d=7\times15=105\), so \(d=35\)