Let $f$ be a function which is continuous and differentiable for all $x$. If $f(1) = 1$ and $f'(x) \leq 5$ for all $x$ in $[1, 5]$, then the maximum value of $f(5)$ is
Step 1: Mean Value Theorem
$\frac{f(5) - f(1)}{5 - 1} = f'(c)$ for some $c \in (1, 5)$.
Step 2: Inequality
Since $f'(x) \leq 5$, then $\frac{f(5) - 1}{4} \leq 5$.
Step 3: Solve
$f(5) - 1 \leq 20 \implies f(5) \leq 21$.
The maximum value is 21.
Final Answer: (D)