9.) \( 40 \% \) of what number is 15 ?

Let's solve for the number! To find out what number \( x \) give us 15 when we take 40% of it, we can set up the equation: \( 0.4x = 15 \). To isolate \( x \), divide both sides by 0.4. That's \( x = \frac{15}{0.4} \), which simplifies to \( x = 37.5 \). So, the number you're looking for is 37.5! Now, here's a fun nugget: Did you know percentages have roots in ancient mathematics? The concept stems from the Latin phrase "per centum," meaning "by the hundred." That means percentages are essentially all about scaling quantities based on a hundred - and they have been used for hundreds of years in trade and economics! For practical uses, understanding percentages is crucial in daily life. When shopping, for instance, knowing how to calculate discounts can help you save money. If an item costs $50 with a 20% discount, you'd quickly figure out you save $10, making the final price $40. It's a handy skill that can keep more cash in your pocket!