Chapter 1 - Section 1.3 - Applications of Linear Equations - 1.3 Exercises - Page 74: 5

D. 40x

We will use x for the unknown number, such that "40% of a number" is 40% $\times$ $x$. We therefore need to consider that 40% as a decimal number can be written as 0.4 and 0.40, and as a fraction it can be written $\frac{2}{5}$. Therefore, 40x is the only invalid form of "40% of a number."