The voltage, $V$ (in volts), across a circuit is given by Ohm's law: $V = IR$, where $I$ is the current (in amps) flowing through the circuit and $R$ is the resistance (in ohms). If we place two circuits, with resistance $R_1$ and $R_2$, in parallel, then their combined resistance, $R$, is given by
$$\frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2}.$$
Suppose the current is 2 amps and increasing at 0.04 amps/sec and $R_1$ is 4 ohms and increasing at 0.5 ohms/sec, while $R_2$ is 3 ohms and decreasing at 0.2 ohms/sec. Calculate the rate at which the voltage is changing.

volts/sec