Two cars approach a stationary observer from opposite sides as shown in the figure.

The observer hears no beats. If the frequency of the horn of the car B is 504 Hz, then the frequency of the horn of the car A will be

The vibrations of a string of length 60 cm fixed at both ends are represented by the equation

$$y = 4\sin \left( {{{\pi x} \over {15}}} \right)\cos (96\pi t)$$

where x and y are in cm and t is in second. Calculate the velocity of the particle at x = 7.5 cm at t = 0.25 s.