2016 AMC 10A Problem 4

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