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This question was asked in IITB (RA) 2016 admissions.

Three person A, B and C each have a bag of five different coloured balls. All three bags have balls from same five colours. A grabs B and C's bag and took two balls without looking and put them in his bag. Then B grabbed A and C's bag and took two balls and put them in his bag, without looking. Then C grabbed A and B's bag and took two balls and put them in his bag without looking. What is the probability that all three bags have different coloured balls?
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1 Answer

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Say there are 5 different coloured ball in each bag (C1,C2, C3, C4, C5)

Now make them color

C1=${\color{Blue} Blue}$

C2=${\color{Green} Green}$

C3=${\color{Yellow} Yellow}$

C4=${\color{Orange} Orange}$

C5=${\color{Red} Red}$

:)

Now, how can we select balls?

Case 1:Say, A can select 1 ball from 5 colors (say C1) from B and other than that color it selects ball from C ( say C2)

Case 2: Say, A can select 1 ball from 5 colors (say C1) from B and same color ball from C

So, what will be selecting possibility for A?

$\binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1$

Now, we should calculate selecting possibility of B

Case 1: Now, B can select any ball from 5 balls(may be same color ball which A chooses) from A, say C1 and other any color say C2 from C

Case 2: B can select any ball from 4 balls(may be same color ball which A chooses) from A, say C1 and again same color ball from C

Case 3:  B can select any ball from 4 balls( other color ball which A chooses) from A, say C2 and  other color ball from C , say C3

Case 4:B can select any ball from 4 balls( other color ball which A chooses) from A, say C2 and again same color ball from C , say C2

So, selection will be $\left ( \binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1+\binom{4}{1}\times \binom{3}{1}+\binom{4}{1}\times 1\right )$

Now, selecting possibility of C

Case 1: Now, C can select any ball from 5 balls(may be same color ball which A chooses) from A, say C1 and other any color say C2 from B

Case 2: C can select any ball from 4 balls(may be same color ball which A chooses) from A, say C1 and again same color ball from B

Case 3:  C can select any ball from 3 balls( other color ball which A and B chooses) from A, say C3 and  other color ball from C , say C4

Case 4:C can select any ball from 3 balls( other color ball which A and B chooses) from A, say C3 and again same color ball from C , say C3

$\binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1+\binom{3}{1}\times \binom{2}{1}+\binom{3}{1}\times 1$

Similarly again A chooses

Case 1: Now, A can select any ball from 5 balls(may be same color ball which A chooses) from B, say C1 and other any color say C2 from C

Case 2: A can select any ball from 4 balls(may be same color ball which A chooses) from B, say C1 and again same color ball from C

Case 3:  A can select any ball from rest 2 balls( other color ball which A and B and C already chooses) from B, say C4 and  other color ball from C , say C5

Case 4:A can select any ball from rest 2 balls( other color ball which A and B and C already chooses) from B, say C4 and again same color ball from C , say C4

Now, what is the selecting possibility

$\binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1+\binom{2}{1}\times 1+\binom{2}{1}\times 1$

Lastly if again B chooses to arrange last 1 ball can select like this

$\left (\binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1+\binom{1}{1}\times 1 \right )$

So, total selection possibility

$\left ( \binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1 \right )$+$\left ( \binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1+\binom{4}{1}\times \binom{3}{1}+\binom{4}{1}\times 1\right )$+$\left ( \binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1+\binom{3}{1}\times \binom{2}{1}+\binom{3}{1}\times 1 \right )$+$\left ( \binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1+\binom{2}{1}\times 1+\binom{2}{1}\times 1 \right )$+$\left (\binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1+\binom{1}{1}\times 1 \right )$

Now, here combination of A,B, C is possible , as A can select first or B can select first or C can select first

in 3!=6 ways

So, no of selection will be

6*($\left ( \binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1 \right )$+$\left ( \binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1+\binom{4}{1}\times \binom{3}{1}+\binom{4}{1}\times 1\right )$+$\left ( \binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1+\binom{3}{1}\times \binom{2}{1}+\binom{3}{1}\times 1 \right )$+$\left ( \binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1+\binom{2}{1}\times 1+\binom{2}{1}\times 1 \right )$+$\left (\binom{5}{1}\times \binom{4}{1}+\binom{5}{1}\times 1+\binom{1}{1}\times 1 \right )$)

edited by

4 Comments

One thing though they have not mentioned

but we have to assume, that it is not a one round game.

means A,choose, then B, then C, Again A choose..........like that goes on.rt?
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@Akriti chk now
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one ball each or 2 balls from a single bag?
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