A listener at rest (with respect to the air and the ground) hears a signal of frequency f r from a source moving toward him with a velocity of 15 m/s, due east. If the listener then moves toward the approaching source with a velocity of 25m/s, due west, he hears a frequency f2 that differs from f1 by 37 Hz. What is the frequency of the source? (Take the speed of sound in air to be 340 m/s.)