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There are 3 black and 5 white balls in a box. Two balls are drawn

randomly one by one without replacement. Find the probability of both
the balls are of same colour.

User Imantha
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1 Answer

3 votes

Answer:5/8) * (4/7) + ( 3/8)* (2/7) = 13/28

Explanation:

The explanation for my answer could go a bit longer but it’s pretty easy to understand the concept & a good approach to hit such questions right in few seconds. Hope it helps !!!!!!

(Remember: we always multiply for AND case & do addition for OR case)

Selection would be in 2 ways possible i.e ( WHITE AND WHITE ball “OR” BLACK AND BLACK ball ) because we have been asked to choose two balls of same color. You cant pick White & White AND Black & Black because in this way u ll end up picking 4 balls which is wrong. so we used “OR”

Now,,,,,Think it like , what is the probability of getting 1 white ball on your first selection out of all balls….???

(Most imp approach)- Don’t consider both selection only in one go !!! so just think it one by one. …..O.K!

Hopefully u too thought it = 5/8 ( i.e there are 5 white balls and you selected one from total 8 balls) ……………………………………….GOOD :)

Now, what about the next ball being white…………..is the most common mistake many could do.

Problem is we have 5 white balls & we have chosen 1 ball out from that,,,, Right?

So, there are only 7 balls left for next selection & How many are there white balls left to select now??? = 4 ( because we have already selected one, out from 5)….. :)

2. PROBABILITY OF GETTING WHITE BALL AGAIN IN SECOND CHANCE = 4/7

PROBABILITY OF GETTING BOTH WHITE = (5/8) * (4/7) (remember, getting the first ball changes the probability of another, since we wanted a white AND a white ball so we ll multiply them both )

3. Similarly, for black we have 2 scenarios, hopefully u thought =(3/8)

4. Can u tell me probability of getting another black now???? = (2/7) ( LIKE WE DID FOR WHITE BALLS)

ANSWER……Probability of getting white & white OR black & black =

= (5/8) * (4/7) + ( 3/8)* (2/7) = 13/28 (& - multiply, OR -addition )

Hope, my long explanation helped you to understand the easy approach for such questions. :)

Thanks for reading! :) have a good day :)

User Tomasantunes
by
7.9k points

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