A bowl contains a total of 200 marbles, of which 100 are red and 100 are blue.

Question

If no marbles are added, how many marbles should be removed so that the probability of drawing a blue marble at random from the bowl is 4/5?

Answer 1

The easiest correct answer is take out 96 of the blue marbles and 99 of the red marbles. Thus it would be left with 4 blue in a total of 5 marbles. Simple - probability is 4/5.

However, if the condition is that MINIMUM how many marbles should be removed to make the probability 4/5, then we must try without removing any blue marble.

Then for each 4 blue marbles there should be 1 red.

Therefore, for 100 blue marbles there should be 25 red. That is 100 blue in 125 total marbles (4:5)

Ans: Remove 75 red marbles. (not 80, Sam :))

Answer 2

100/200=blue; to get

4/5=blue; so pull 80 from the red=80/100=4/5.