In a situation that involves percentages, there are often three questions we are interested in answering.

Suppose a tank is filled with some water.

1. If we know that the tank is 25% filled and can hold 36 liters, we can ask: What is 25% of 36 liters?
2. If we know that the tank has 9 liters and is 25% filled, we can ask: How many liters are in a full tank?
3. If we know that the tank has 9 liters but can hold 36 liters when full, we can ask: What percentage of 36 liters is 9 liters?

We can use a double number line diagram, a table, or a tape diagram to help us reason about each question.

We can also use our knowledge of fractions or relationships between numbers. For instance, we know that 9 is $\frac{1}{4}$ of 36, or $36 \div 4$, so it is $\frac{1}{4}$ of 100% or 25%.

In general, in a situation where $A$ of $B$ is $C$, we can find the value of $A$, $B$, or $C$ if we know the other two values.