1. **State the problem:** We are given a function $f(x) = 10x + 13$ and need to find the value of $x$ such that $f(x) = 40$. 2. **Write the equation:** Set the function equal to 40: $$10x + 13 = 40$$ 3. **Isolate $x$:** Subtract 13 from both sides: $$10x = 40 - 13$$ $$10x = 27$$ 4. **Solve for $x$:** Divide both sides by 10: $$x = \frac{27}{10} = 2.7$$ 5. **Answer:** The value of $x$ for which $f(x) = 40$ is $2.7$. This means when you input $2.7$ into the function, the output will be 40.