Answer:
4 tokens.
Step-by-step explanation:
The expected value of a given event is calculated as:
EV = (x₁*p₁ + x₂*p2 + ... + xₙ*pₙ)
Where xₙ is the n-th outcome, and pₙ is its probability.
In this case, our experiment is:
You draw two times.
We have 30 cards with no prize
We have 10 cards with a prize.
A total of 40 cards.
As we draw two times (and the first time we draw a card we put it back in the deck) we can consider the events as independent, so we can find the expected value per draw.
Now we can define:
x₁ = drawing a blank card = 0 tokens
The probability will be equal to the quotient between the number of blank cards and the total number of cards
p₁ = 30/40 = 3/4
x₂ = drawing a prized card = 8 tokens.
The probability will be equal to the quotient between the number of prized cards and the total number of cards:
p₂ = 10/40 = 1/4
Then the expected value per draw is:
EV = ( (3/4)*0 tokens + (1/4)* 8 tokens) = 2 tokens.
And we have two draws, then the expected value of two draws is two times the expected value per draw, this means that the expected value in our case is:
expected value = 2*(2 tokens) = 4 tokens.
The correct option is the first one, counting from the top.
