Here we know the K.E of the neutrons i.e we know their velocity , so you can calculate that how much time will they take to reach 10m
Now as T1/2 is given so we know the decay constant , so we can calculate N/No from 1st order equation because decay const and time (to reach 10m) are known
Thanks I got it. I got confused as I was thinking mass is changing with time so velocity will vary . From option it become clear that mass change is not significant.