The function f is defined by f(x) = ax + b. If f(1) = 5 and f(3) = 13, what is the value of a + b?

Use the two function values to write equations: $f(1) = a + b = 5$ and $f(3) = 3a + b = 13$. Subtract the first from the second: $2a = 8$, so $a = 4$, and then $b = 1$. The question asks for $a + b$, which is $4 + 1 = 5$, choice D. In fact, $a + b$ is just $f(1)$, which was given directly, so the work above is a check. Choice A (8) is not supported by either equation. Choice B (4) is only the value of $a$. Choice C (18) is $13 + 5$, adding the function values. Read the question carefully before solving.