doppler's effect??

Since it is the sound soure that is moving, use the Doppler Effect formula, the apparent frequency f is:

f = (fo)[c/(c-v)]
where fo is the original frequency of sound
c is the speed of sound in air
v is the speed of the sound source (+ve if moving towards the observor, and -ve if moving away from the oberveor)

Hence, 80 = (fo)[343/(343-v)]
and 60 = (fo)[343/(343+v)]

Dividing, 80/60 = (343+v)/(343-v)
i.e. v = 49 m/s