The problem asks to find a number $x$ such that the ratio $72:x$ is equivalent to the ratio $12:3$.

1. Problem Description

The problem asks to find a number $x$ such that the ratio $72:x$ is equivalent to the ratio $12:3$.

2. Solution Steps

We are given the ratio $12:3$ and we want to find a number $x$ such that $72:x$ is equivalent to $12:3$. We can write this as a proportion:

$\frac{72}{x} = \frac{12}{3}$

First, we can simplify the ratio $12:3$ by dividing both sides by 3:

$\frac{12}{3} = \frac{12 \div 3}{3 \div 3} = \frac{4}{1}$

So now we have:

$\frac{72}{x} = \frac{4}{1}$

To solve for $x$, we can cross-multiply:

$72 \cdot 1 = 4 \cdot x$

$72 = 4x$

Divide both sides by 4:

$x = \frac{72}{4}$

$x = 18$

Alternatively, we can observe that to get from 12 to 72, we multiply by 6 since $12 \times 6 = 72$. Thus we need to multiply the second number in the ratio $12:3$ by 6 to find the equivalent ratio. So, $3 \times 6 = 18$. Therefore, the ratio $72:18$ is equivalent to the ratio $12:3$.

3. Final Answer

$18$