A stationary source is emitting sound at a fixed frequency f0, which is reflected by two cars approaching the source. The difference between the frequencies of sound reflected from the cars is 1.2 Hz.

Let the speed of the cars be v1 and v2.
Frequency received by the car, f1 = (v/(v-v1))f0
Frequency reflected by car, f2 = ((v+v2)/(v))(v/(v-v1))f0
Δf = f'2 - f1 = ((v+v2)/(v-v1) - v/(v-v1))f0
Solving we get:
Δf = (2vv2/(v-v1))f0
Equating this Δf to 1.2f0/100 we get, v2 - v1 = 7