Let the speed of X is 10m/s then A will Complete the race in 10 Secs.

Now X beat Y by 10m , so Y was at 90m mark when X completed the race i.e

Y covered 90 m in 10 secs so speed of Y is 9m/s. So now Y will take additional

10(m) / 9(m/s) secs i.e 1.11 secs to complete the race.

Y beat Z by 10 m so Z covered 90 m in 10 + 1.11 secs

speed of Z is  90/11.11 = 8.1m/s now he will also take additional 10/8.1 secs

i.e. 1.234 sec to cover next 10 m.

Now Z will reach 100 m 1.11 + 1.234  = 2. sec after X

Thats why X beat Z by 10 * 2.344 = 23.44 m