Question

What is the fourth proportional to 5, 8, and 15?
(A) 18
(B) 24
(C) 19
(D) 20

Answer 1

Hint: Main hint for such type of question is that we have to keep in mind that we have to keep the fourth proportion to be x (which we have to calculate) and just substitute that in the equation, like say for a, b, c and d; equation for them being in proportion is $\dfrac{a}{b} = \dfrac{c}{d}$.

Complete step-by-step answer:

If we just relate the values we have been given in the question to a, b, c and d. Let us assume d to be x and a, b and c to be 5, 8 and 15 respectively.
Substitute them in the formula as described in the hint, we have
$\dfrac{5}{8} = \dfrac{{15}}{x}$
On Cross multiplying we get,
$5 \times x = 15 \times 8$
$5 \times x = 120$
$x = \dfrac{{120}}{5}$
$x = 24$
Option B. is the Correct Answer.

Note: For questions related to proportion just keep in mind the basic definition of proportion which is A proportion is simply a statement that two ratios are equal. It can be written in two ways: as two equal fractions $\dfrac{a}{b} = \dfrac{c}{d}$, or using a colon, $a:b = c:d$.