For constant speed v = d/t where d is the distance travelled and t is the time it takes to travel it. Let's say that the bike travelling South covered a distance of x km and the bike travelling North has covered y km. Then:
x + y = 40.25
From the speed equation for each biker:
v = x/1.75
v + 5 = y/1.75
We want to find v.
So take the first equation and solve it for y:
y = 40.25 - x
and put this into the third equation:
v + 5 = (40.25 - x)/1.75
Now solve the second equation for x:
x = 1.75*v
and put this into the last equation we just got:
v + 5 = (40.25 - [1.75*v])/1.75
This is one equation in one unknown, which you can solve. I got v = 9 km/h. If you need help solving the equation, just let me know.