Welcome back to Percent Foundations for Everyday Life! We have arrived at Lesson 8 of 8 — the final lesson in this course. Before we dive in, take a moment to appreciate how far we have come. We started by understanding what a percent even means, moved through conversions and mental math shortcuts, and then learned to calculate a part from a percent and to figure out what percent one number is of another.

Today we close the loop by tackling the third and final type of percent problem: finding the whole when we know only the part and the percent it represents. Imagine someone tells us, "I saved $45, and that was 15% of the original price." We know the piece and the rate, but the original price is missing. By the end of this lesson, we will have a clear method for recovering that missing whole every time.

Every percent problem involves three quantities: the part, the percent (rate), and the whole (base). In each problem, two of these are given and one is missing. Across the last three lessons of this course, we have met all three setups:

• Lesson 6 — Part missing: What is 20% of 150? → Multiply.
• Lesson 7 — Percent missing: 18 is what percent of 25? → Divide part by whole, then convert.
• This lesson — Whole missing: 45 is 15% of what number? → Divide part by decimal percent.

Recognizing which piece is missing is the single most important step before we do any arithmetic. Once we know which setup we are dealing with, the correct operation follows naturally.

In Lesson 6, our core relationship was: