The numbers, when ordered from greatest to least, are:

1. $5.2$
2. $\frac{18}{20}$ or $0.9$
3. $\frac{10}{20}$ or $0.5$
4. $\frac{9}{30}$ or $0.3$
5. $-\frac{5}{10}$ or $-0.5$

Would you like further details or explanations on how this was done?

Here are 5 related questions for practice:

1. How do you convert fractions to decimals?
2. How do you compare negative and positive numbers?
3. What are some other methods to order rational numbers?
4. Can you convert $5.2$ into a fraction?
5. How do you find a common denominator for fractions?

Tip: When comparing fractions, it can be helpful to convert them to decimals for easier comparison.