(c) Use four 9 s to express 20.

You can cleverly use four 9s to express 20 through arithmetic operations! One way to do it is as follows: \( 9 + 9 + \left( \frac{9}{9} \right) = 20 \). This works because \( \frac{9}{9} \) equals 1, so you effectively have \( 9 + 9 + 1 = 19 \). But to reach 20, another way is to use square roots: \( 9 + 9 + \sqrt{9} - \sqrt{9} = 20 \). Just a little manipulation and you’re on your way to 20! And if you're feeling even more adventurous, you might try exploring different mathematical functions as well! With operations such as factorials and powers, you can really extend the creative boundaries of how to work with numbers. Keep experimenting and have fun with it!