2016 AMC 12A Problem 3

The remainder can be defined for all real numbers x and y with y \neq 0 by \text{rem} (x ,y)=x-y\left \lfloor \frac{x}{y} \right \rfloorwhere \left \lfloor \tfrac{x}{y} \right \rfloor denotes the greatest integer less than or equal to \tfrac{x}{y}. What is the value of \text{rem} (\tfrac{3}{8}, -\tfrac{2}{5} )?

\textbf{(A) } -\frac{3}{8} \qquad \textbf{(B) } -\frac{1}{40} \qquad \textbf{(C) } 0 \qquad \textbf{(D) } \frac{3}{8} \qquad \textbf{(E) } \frac{31}{40}

Full credit goes to MAA for authoring these problems. These problems were taken on the AOPS website.