Question

Find the fourth proportional to 3, 8 and 21.

Answer 1

Let the fourth proportional be $$x$$.

Then, $$\text{Product of extremes} = \text{Product of means}$$.

$$3 : 8 :: 21 : x$$

$$3 \times x = 8 \times 21$$

$$x = \frac{8 \times 21}{3} = 56$$