first you need to find how many points he scored for C, which we know is worth 15 points each based off the diagram. you know he scored 6 baskets and each are worth 15 so you do 15*6 which gets you 90 points. this goes in the score for C. Next you find out how much is the score for B. You already know it’s worth a total of 140 and you have already score 20 and 90 so you just subtract those numbers from the total of 140. you should get 30 points left and this goes to your score for B. To find out how many baskets he’s score for C, knowing that each are worth 10 points and he’s scored 30 points, you do 30/10 which gets you 3.
i hope this helps!