Two point charges are located on the x axis. The first is a charge +Q at x = -a. The second is an unknown charge located at x = +3a. The net electric field these
charges produce at the origin has a magnitude of 2kQ/a>. What are the two
possible values of the unknown charge?

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