Find the fourth proportional to the numbers 15, 45 and 75.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 225
The fourth proportional to \(a,\ b,\ c\) is the number \(d\) with \(a:b=c:d\), so \(\displaystyle d=\frac{b\times c}{a}\).
\(15:45=75:d\)
\(\displaystyle d=\frac{45\times75}{15}=3\times75=225\)
The fourth proportional is 225.