In a game of 90 points A can give B 15 points and C 30 points. How many points can B give C in a game of 100 points?
While A scores 90 points, B scores $\left(90-15\right)$=75 points and C scores $\left(90-30\right)$= 60 points
i.e., when B scores 75 points, C scores 60 points
$\text{=> When B scores 100 points, C scores }\dfrac{60}{75}\times 100\text{ = 80 points}$
i.e., in a game of 100 points, B can give C $\left(100-80\right)$=20 points