In a race of 200m, B gives a start of 20m to A and C gives a start of 30m to B. The start, that C can give to A in the same race.

47m

1st condition:

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

2nd condition:

Ratio of distance covered ⇒ B : C = 170 : 200 = 17 : 20

Now,

   A    :   B    :   C

  9    :   10

             17  :   20

153   :  ----  :   200

Here we can see that, if C covers 200 m then A covers 153 m.

Therefore,

C gives ⇒ 200 - 153 = 47m start to A in the same race.