Two trains are moving towards each other at speed of 144 km/hr and 54 km/ hr relative to the ground. The second sounds a whistle of frequency 710 Hz, the frequency of this whistle as heard by a passenger in the first after the trains crossed each other is $x\times {10}^2 \mathrm{Hz}$,  the value of x is $(\mathrm{v}=340\mathrm{m}/\mathrm{s})$