The ratio of $15$ to $5$ can be expressed as $3:1$.

To clarify this concept further, let’s break it down. A ratio is a method of comparing two quantities by indicating how many times one quantity contains or is contained within the other. Here, we are comparing $15$ to $5$. To determine the ratio, we divide both numbers by their greatest common divisor (GCD). In this case, the GCD of $15$ and $5$ is $5$.

When we divide $15$ by $5$, we obtain $3$. Likewise, dividing $5$ by $5$ results in $1$. Therefore, the ratio of $15$ to $5$ simplifies to $3:1$, which indicates that for every $3$ units of the first quantity, there is $1$ unit of the second quantity.

Ratios are commonly encountered in various real-life scenarios, such as in cooking recipes, mixing paints, or comparing speeds. Mastering the simplification and interpretation of ratios is an essential skill in GCSE Maths, as it aids in solving problems efficiently and accurately. Always remember to identify the greatest common divisor to reduce ratios to their simplest form.