Let $X_{BC}$ be the distance between towns B and C. Then: $$X_{BC}=64km/h(\frac {45min}{60min/h})=48km$$
So, it takes C 20 minutes to get from B to A. Let $V_c$ be the speed of C, and $t=20min=1/3h$. Then:
$$X_{AB}=V_c t=\frac {V_c}{3}h$$
Knowing that the time that takes A to get to B ($t_{AB}$) is equal the time that takes C to get to B ($t_{BC}$). Hence, $t_{AB}=t_{BC}$:
$$\frac {X_{AB}}{V_a}=\frac {X_{BC}}{V_c}$$ $$\frac {\frac {V_c}{3}h}{64km/h}=\frac {48km}{V_c}$$ $$\frac {V_c^2}{3}=3072km^2/h^2$$ $$V_c^2=9216km^2/h^2$$ $$V_c=96km/h$$