Question

What is the fourth proportional to 5, 18, 15?

Answer 1

Hint: In this question, we need to find the fourth proportional to a given sequence of three numbers. Thus, we need to first know about the definition of fourth proportional and then use the numbers given in the question to obtain their fourth proportional.

Complete step-by-step answer:
The fourth proportional of three numbers a, b, c is defined to be d if
$\dfrac{a}{b}=\dfrac{c}{d}................(1.1)$
Here, the values of a, b and c can be taken to be a=5, b=18, c=15. Now, if their fourth proportional is denoted by d, then it must satisfy equation (1.1). Thus, we obtain the value of d by the following calculations
$\dfrac{a}{b}=\dfrac{c}{d}\Rightarrow d=\dfrac{c\times b}{a}$
Now, using the values of a, b and c to be 5, 18 and 15 respectively, then
$d=\dfrac{15\times 18}{5}=3\times 18=54$
Thus, from the above calculations, we obtain the fourth proportional of 5, 18, 15 as 54.

Note: In this question, we should be careful while counting the fourth proportional and should remember that the ratio of the first two terms should be equal to that of the last two terms but taken in order, i.e. the ratio of a and be should be equal to the ratio of c and d but not equal to the ratio of d and c. Also, we could have any variable names in solving the question as ultimately they would be replaced by the given numbers in the calculation.