In a race of 200m, B gives a start of 10m to A and C gives a start of 20m to B. The start, that C can give to A in the same race.

29 m

1st condition:

Ratio of distance covered ⇒ A : B = 190 : 200 = 19 : 20

2nd condition:

Ratio of distance covered ⇒ B : C = 180 : 200 = 9 : 10

Now,

   A    :   B    :   C

  19   :   20

             09  :   10

171   :  ----  :   200

Here we can see that, if C covers 200 m then A covers 171 m.

Therefore,

C gives ⇒ 200 - 171 = 29m start to A in the same race.