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130 are total balls out of which 30 are black and 100 are red, so if you need to get 0.5 probability black balls should be equal to red balls ie 70 more black balls are needed to get probability of 0.5

increasing the probability of picking back to 0,5 means it wil be as likely you pick black as you pick red. So there needs to be an equal chance of picking red or black, in this case, To achieve this, you will need to add 70 black balls to the 30 in the pot, to match the 100 red balls already in the pot.

Jessicahannah is the only person who is replied correctly.

The mistake others were making is forgetting that when adding black balls, we increase the total number of balls. 

It always helps if you simplify the numbers in a problem. Imagine we were told that there were 8 black and red balls, and 3 were black. How many black balls would we need to add? At the moment, we have 3/8. So if we add one we will have half? 4/8? No, we will have 4/9. 

So we must first work out how many red balls there are: 8 - 3 = 5. Now we can add 2 black balls to our 3 original balls, so we have 5. Now there are 10 balls total, 5/5. 

Jessicahannah demonstrates this method well with the original question.


we need to add 35 black balls to increase probability of picking black ball to 0.5

Required probability=0.5

as 30 balls are black so probability= 30/130 now it increase it to 0.5 to add x black ballso

(30+x)/130=0.5

30+x=130*0.5

30+x=65

x=35

so we would add 35 black ball to getting probability 0.5

The probability of 0.5 is a half, this means 1/2 of the balls must be black. In order to figure this out we need to know how many red balls there are, so this means we subtract 30 (number of black balls) from 130 (total amount) . The awnser is 100 (amount of red balls) since we  need black and red balls to be the same amount we will need to take 30 away from 100 to find out how many more black balls will make it equal. So since 100-30 = 70. You will need to add 70.

So if the total number i.e. 1 is = 130, then to have a value of 0.5, half of 1, you need to half 130 balls. This is 65. Therefore for there to be 0.5 probability of picking black, there must in total be 65 black balls. 

We already have 30 balls, so how many more do we need to get to 65?

65-30 = 35. You need to add 35 more black balls.

There are 130 black and red balls in a pot. If 30 are black, how many black balls will you need to add to the box to increase the probability of picking a black to 0,5?
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