To solve the equation $x + 3y = 16$ for $x + y$, we need to express $x$ in terms of $y$ and then substitute into the second equation.

1. Start with the given equation: $x + 3y = 16$
2. Solve for $x$: $x = 16 - 3y$
3. Now, substitute $x = 16 - 3y$ into the equation $x + y$: $x + y = (16 - 3y) + y$
4. Simplify the equation: $x + y = 16 - 3y + y = 16 - 2y$

So, $x + y = 16 - 2y$.

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5 Related Questions:

1. How would you solve $x + y = 10$ if given a different linear equation, like $2x + 5y = 20$?
2. What are the steps for solving a system of linear equations with two variables?
3. How can you use substitution and elimination methods to solve simultaneous equations?
4. What does the solution to a system of linear equations represent geometrically?
5. How do you interpret the solution $x = 16 - 3y$ in the context of the original equation?

Tip: When solving for one variable in terms of another, always isolate the desired variable first and then substitute it into the related equation. This method is known as substitution.