Expressing one quantity as a fraction of another simply means writing a given quantity in a fraction form and reducing the fraction to its lowest term. For instance, expressing one quantity of A as a fraction of another quantity B is written as A/B and then reduce to its simplest form.

In expressing one quantity as a fraction of another we make sure all the quantities are written in the same units.
Steps in Expressing one quantity as a fraction of another
Follow the steps listed below in expressing one quantity as a fraction of another
Step 1: Ensure the quantities have the same units of measure
Step 2: if the quantities are not of the same units, chose a quantity you find easier to work with and write them in the same units.
Step 3: write the first quantity as the numerator and the second quantity as the denominator. Note that the quantity that follows “of” is the denominator.
Step 4: simplify the fraction.
Let’s look at the example below
\text{Example: Express 40cm as a fraction of 80cm.}\\ Answer: \text{ Expressing the quantity 40cm as a fraction of 80cm} \space=\frac{40}{80}=\frac{1}{2}
This is pretty good, right? Let’s look at another example
\text{Example: Express 300cm as a fraction of 0.6m }\\ Answer: \text{First change the 300cm to 0.3m or 0.6m to 600cm} \space=\frac{300}{600}=\frac{3}{6}=\frac{1}{2}
On equivalent fractions click here