Question 310686
v=speed
s=the distance
t=time

now the speed of the first train is v1=35km/h, the speed of the second train is 65km/h.

The distance for the first train is s1=v1*t and for the second train s2=v2*t
The distance betwen them will be s=s1+s2, s=(v1*t)+(v2*t), s=(v1+v2)*t

t=s/(v1+v2)=350km/(65km/h+35km/h)=3.5hours