Answer
$$\approx -0.27 L/s$$
Work Step by Step
We need to apply chain rule as follows:
$$\dfrac{dV}{dt}=(\dfrac{\partial V}{\partial P})(\dfrac{dP}{ dt})+(\dfrac{\partial V}{\partial T})(\dfrac{dT}{dt})\\ =(-8.31) \times \dfrac{T}{P^2} \times \dfrac{dP}{ dt} +8.31 \times (\dfrac{1}{P}) \times \dfrac{dT}{dt} \\=(-8.31) \times [-\dfrac{320}{(20)^2}] (0.05)+(\dfrac{1}{ 20}) \times (0.15)] \\ \approx -0.27 L/s$$
