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YOU ROLL A FAIR SIX-SIDED DIE.

  • IF THE RESULT IS EVEN, YOU DRAW A BALL FROM URN A.
     
  • IF THE RESULT IS ODD, YOU DRAW A BALL FROM URN B.
     

URN A CONTAINS $3$ BALLS MARKED "$10$" AND $1$ BALL MARKED "$0$". URN B CONTAINS $2$ BALLS MARKED "$5$" AND $6$ BALLS MARKED "$1$".

WHAT IS THE EXPECTED VALUE OF THE NUMBER ON THE BALL YOU DRAW?
 

  1. $3.75$
     
  2. $4.75$
     
  3. $5.00$
     
  4. $6.25$

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Urn vs Dice table
No. on die's face$1$$2$$3$$4$$5$$6$
Urn SelectedBABABA

Additionally, Urn A has labels $\left \{\underbrace{10}_\text{3 balls}, \underbrace{0}_\text{1 ball} \right \}$ and Urn B has labels $\left \{\underbrace{5}_\text{2 balls}, \underbrace{1}_\text{6 balls} \right \}$ and $P(\text{odd face})=P(\text{even face})=\frac{1}{2}$ 

Let $B10, B0, \cdots$ denote the event of the listed labelled ball being drawn.



Probability of drawing balls = $\dfrac{\text{required balls}}{\text{total no. of balls in that urn}}$
So,
$$P(B10)= P(\text{even face}) \times \frac{3}{4} = \frac{1}{2}\times \frac{3}{4}=\frac{3}{8}$$

$$P(B0)= P(\text{even face}) \times \frac{1}{4} = \frac{1}{8}$$

$$P(B5)= P(\text{odd face}) \times \frac{2}{8} = \frac{1}{8}$$

$$P(B1)= P(\text{odd face}) \times \frac{6}{8} = \frac{3}{8}$$
 


Hence the expectation would be:

$$\boxed{\mathbb{E}(\text{die face})=\dfrac{1}{8} \left [ (10 \times 3) + 0 + 5 + 3 \right ] = \frac{38}{8} = 4.75}$$

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