Solve the Ratio Equation: Finding the Missing Number in 20:?=1

To solve this problem, we'll determine the number $a$ such that $20 : a = 1$.

By definition of division, for the expression $20 : a = 1$ to hold true, $a$ must be equal to the dividend when the division results in 1.

In mathematical terms, this means that dividing a number by itself yields 1. Therefore, the only value that correctly satisfies $20 : a = 1$ is $a = 20$.

This approach uses the fundamental property of division that dividing any non-zero number by itself results in 1, confirming that:

• If $a = 20$, the equation becomes $20 : 20 = 1$, which is true.

Thus, the number that makes the equation true is $20$.

The correct answer from the choices provided is option 3: $20$.