The given sentence is: "Four times the sum of a number $x$ and a number $y$ is equal to 20."

We need to express this sentence mathematically.

• "The sum of a number $x$ and a number $y$" is written as $(x + y)$.
• "Four times" this sum is represented as $4(x + y)$.
• This is equal to 20.

Thus, the equation is:

$4(x + y) = 20$

The correct answer is C.

Would you like more details on solving this equation, or help with similar problems? Here are 5 related questions to expand your understanding:

1. How would you solve the equation $4(x + y) = 20$ for $y$ if $x$ is given?
2. How do you distribute a number over parentheses in an equation like $4(x + y)$?
3. What would happen if you change "sum" to "product" in the original sentence?
4. Can you explain how to graph the equation $4(x + y) = 20$?
5. How do you verify the solution to a word problem involving algebraic expressions?

Tip: When interpreting math word problems, always look for key words like "sum," "times," or "equal to" to correctly translate them into equations.