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Bag A contains 2 unbiased and 3 biased coins whereas Bag B contains 3 unbiased and 2 biased coins. A bag is selected at random and 2 coins are taken out simultaneously. The probability, that both coins are unbiased is:

Solution

✅ Correct Option: 1

Bag A contains 2 unbiased and 3 biased coins (5 coins total).

Bag B contains 3 unbiased and 2 biased coins (5 coins total).

A bag is selected at random, and 2 coins are taken out simultaneously.


Since either bag can be selected:

P(both unbiased)=P(Bag A)×P(both unbiased from A)+P(Bag B)×P(both unbiased from B)P(\text{both unbiased}) = P(\text{Bag A}) \times P(\text{both unbiased from A}) + P(\text{Bag B}) \times P(\text{both unbiased from B})

The probability of selecting each bag is:

P(Bag A)=12P(\text{Bag A}) = \frac{1}{2}

P(Bag B)=12P(\text{Bag B}) = \frac{1}{2}


For Bag A with 2 unbiased coins:

Number of ways to select 2 unbiased coins from 2: (22)=1\binom{2}{2} = 1

Total ways to select 2 coins from 5: (52)=5×42×1=10\binom{5}{2} = \frac{5 \times 4}{2 \times 1} = 10

P(both unbiased | Bag A)=110P(\text{both unbiased | Bag A}) = \frac{1}{10}


For Bag B with 3 unbiased coins:

Number of ways to select 2 unbiased coins from 3: (32)=3×22×1=3\binom{3}{2} = \frac{3 \times 2}{2 \times 1} = 3

Total ways to select 2 coins from 5: (52)=10\binom{5}{2} = 10

P(both unbiased | Bag B)=310P(\text{both unbiased | Bag B}) = \frac{3}{10}


P(both unbiased)=12×110+12×310P(\text{both unbiased}) = \frac{1}{2} \times \frac{1}{10} + \frac{1}{2} \times \frac{3}{10}

=120+320= \frac{1}{20} + \frac{3}{20}

=420= \frac{4}{20}

=15= \frac{1}{5}

Therefore, the probability that both coins are unbiased is 15\frac{1}{5}.

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