Question

A box contains 12 balls out of which x are black . if one ball is drawn at random from the box what is the probability that it will be a black ball. if six more are put in the box

Answer 1

no. of black balls=x
1st the total no. of ball was=12

if one ball is removed and six more is added then the total no. of ball=(12-1)+6=17

therefore probability of getting black ball=x/17

Answer 2

Number of black balls in the box = x
Total number of balls in the box = 12

i.e.,    n(S) = 12

(0 Let A be the favourable outcomes of getting black ball, then
n(A) = x
Therefore,

P(A) = 

(ii) Number of white balls in the box = x + 6 Total number of balls in the box = 12 + 6 = 18. i.e.,    n(S) = 18
Let B be the favourable outcomes of getting new white balls, then
n(B) = x + 6
Therefore,

P(B) = 
Now according to question

P(B) = 2P(A)

      

Hence, the number of white balls = 3.