In a game of carrom of 100 points, A can give B 20 points and C 40 points , then points B can give C is :

25 points

Total points of game $=100$.

A gives B $20$ points.

So when A scores $100$, B scores $80$.

Thus skill ratio:

$A : B = 100 : 80 = 5 : 4$

A gives C $40$ points.

So when A scores $100$, C scores $60$.

$A : C = 100 : 60 = 5 : 3$

Now combine ratios:

$A : B : C = 5 : 4 : 3$

Hence

$B : C = 4 : 3$

When B scores $100$, C scores $75$.

Therefore B can give C $25$ points.

The points B can give C is $25$.