Every object has natural frequencies of oscillation with which they will respond when deflected from equilibrium. The marching soldiers appear to have been marching at a pace such that their stomping boots drove the oscillation of the bridge at :

a resonate  frequency or natural frequency

Explanation:

i think it would be 80 km/h because 30 + 50 = 80 (sorry if this not the right answer i'm trying my best)

when jerome paddle in flowing river towards east the net speed of jerome will increase

so the net speed of jerome will be given as

now the speed of paul will be same as it is given as he is running on the ground

so

so here we can say since the speed of jerome is more so he will win the race

now as we know the distance so the time difference is given as

so jerome will reach first and the time difference will be 203.55 s or 3.4 min