According to the Doppler effect, the apparent frequency $f'$ heard by a stationary observer when the source is moving at an angle to the line joining them is given by considering the component of the source's velocity along the line of sight. The component of the velocity of the source approaching the observer is $v_s \cos \theta = 19.4 \cos(60^\circ) = 19.4 \times \frac{1}{2} = 9.7 \text{ ms}^{-1}$. The formula for apparent frequency when the source approaches a stationary observer is: $f' = f \left( \frac{v}{v - v_s \cos\theta} \right)$ where $v$ is the velocity of sound, $v_s$ is the velocity of the source, and $f$ is the actual frequency. Substituting the given values: $f' = 100 \left( \frac{330}{330 - 9.7} \right)$ $f' = 100 \left( \frac{330}{320.3} \right)$ $f' \approx 100 \times 1.03028 \approx 103 \text{ Hz}$ Thus, the apparent frequency observed is approximately $103 \text{ Hz}$.