This topic is in the 'Ratios and Proportions' section. Ratios are just comparisons of one number to another, and proportions are equations that have ratios on both sides. So if you have twice as many dollars as quarters, then the ratio of dollars to quarters is 2 to 1, and you can write that as 2:1 or 2/1. The cool thing about ratios is that they are general and don't care about how many dollars and quarters you have. So if you had 10 dollars and 5 quarters, or 100 dollars and 50 quarters, the ratio of dollars to quarters would always be 2:1. Sometimes you'll have tricky problems where you need to find out the total amount of something starting from a ratio, and we have some nice example tutorials to guide you through those kinds of problems. You'll also find some great proportion tutorials, and in those you'll learn about cross multiplication, which is one of the most useful tricks out there for quickly solving problems!