Let the velocity of sound be $v = 330 \text{ m/s}$, the velocity of the car be $v_c = 30 \text{ m/s}$, and the actual frequency of the horn be $f = 600 \text{ Hz}$. When the sound travels towards the hill, the car acts as a moving source approaching a stationary reflector (the hill). The apparent frequency $f_1$ received by the hill is: $f_1 = f \left( \frac{v}{v - v_c} \right)$ When the sound is reflected from the hill, the hill acts as a stationary source emitting frequency $f_1$, and the car acts as an observer moving towards the source. The frequency heard by the driver is: $f' = f_1 \left( \frac{v + v_c}{v} \right)$ Substituting $f_1$ into the second equation gives the formula for the apparent frequency of an echo heard by a moving source: $f' = f \left( \frac{v + v_c}{v - v_c} \right)$ $f' = 600 \left( \frac{330 + 30}{330 - 30} \right)$ $f' = 600 \left( \frac{360}{300} \right)$ $f' = 600 \times 1.2 = 720 \text{ Hz}$ Therefore, the driver hears a frequency of $720 \text{ Hz}$.