$\dfrac{\text{number of not-black cars}}{\text{total number of vehicles}}=\dfrac{56}{20+56+12+32}=\dfrac{7}{15}$
Putting it into a calculator, we see that $\dfrac{7}{15}=0.4666...=46.7\%$. Given that $50\%=0.5$, we can see that the relative frequency of not-black cars is close to $50\%$ but not exactly $50\%$, therefore Arthur is not correct.