To find the expected number of white marbles when 20 more marbles are pulled out of the bag, we first need to determine the probability of pulling a white marble based on the given outcomes.
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Total number of marbles: Add up the outcomes for all colors: \[ \text{Total} = 36 , (\text{Blue}) + 42 , (\text{Yellow}) + 18 , (\text{Black}) + 24 , (\text{White}) = 120 \]
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Probability of pulling a white marble: The probability \( P(\text{White}) \) is calculated as follows: \[ P(\text{White}) = \frac{\text{Number of white marbles}}{\text{Total number of marbles}} = \frac{24}{120} = \frac{1}{5} \]
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Expected number of white marbles in 20 draws: Multiply the probability of pulling a white marble by the number of marbles pulled: \[ \text{Expected number of white marbles} = P(\text{White}) \times \text{Number of draws} = \frac{1}{5} \times 20 = 4 \]
Thus, the expected number of white marbles when 20 more marbles are pulled out of the bag is \( \boxed{4} \).