To solve the problem of finding the third proportional of the numbers 9 and 60, let's first define what third proportional means.

In mathematics, if we have two numbers a and b, a third proportional x is a number such that a : b = b : x. This means the ratio of a to b is the same as the ratio of b to x.

Given:

• a = 9
• b = 60

We need to find x such that:

\frac{9}{60} = \frac{60}{x}

Let's solve for x using cross multiplication:

9 \cdot x = 60 \cdot 60

Calculate the right-hand side:

60 \cdot 60 = 3600

The equation becomes:

9x = 3600

Now, solve for x by dividing both sides by 9:

x = \frac{3600}{9}

Calculate the division:

x = 400

Hence, the value of x is 400. Therefore, the correct answer is 400.