Two resistors $R_1$ and $R_2$ are in parallel. If $R_1 = 10\Omega$ and $R_2 = 20\Omega$, and their values are changing at rates $dR_1/dt = 1\Omega/s$ and $dR_2/dt = -1\Omega/s$, what is the rate of change of the equivalent resistance $R_{eq}$?