Answer:
the time when he starts to deaccelerate will be =
distance between a and b at the starting = x-y = 12.6m
Explanation:
writing equation of motion for traveller A v = u + at
where v is the final u the initial velocity and a is the acceleration
u = 10 v= 8 and a = -5 applying the values in the equation we get t = 
therefore athlete A deaccelerates from 10 to 8 m/s in
s
so the time when he starts to deaccelerate will be = 2 - 
=
writing equation of motion for B
v = u + at
where v= 8 u = 0 and t = 2
therefore applying the values a= 4

therefore distance travelled be B in 2
= 
therefore
=x+ 8m
for traveller b distance travelled in 2
= y + 10×
+ 10×
+
-5×
= y+19.6 m
given 
therefore x-y-11.6 = 1
therefore distance between a and b at the starting = x-y = 12.6m