Q3. An urn contains 7 balls, 4 of which are black and the rest white.
(a) Two are drawn together and one is found to be black. What is the probability that the other is also?
(b) Starting with all 7 balls in the urn, balls are drawn randomly without replacement until all 3 white balls are found. What is the probability that the third white ball is found on the
(i) fourth attempt? (ii) seventh attempt?
(a) & there are 3 black balls left And also 3 white So p(ball=black)=3/6=0.5 (b) (i) & w-we pick white ball b- black/ white ball is found on the fourth attempt& so there can be combinations wwbw p=3/7*2/6*4/5*1/4=1/35 wbww& p=3/7*4/6*2/5*1/4=1/35 bwww p=4/7*3/6*2/5*1/4=1/35 P=3/35 (ii) The prob. Is absolutely the same as if we pick the last ball,and it should be white P(ball=white)=3/7