The Probability of a Blue Ball

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A box contains b blue balls and r red balls. If 4 blue balls and 3 red balls are added to the box, and if one ball is obtained at random from the box, then the probability that a blue ball is obtained can be represented by

Review: The Probability of a Blue Ball


Explanation

If we get confused by the algebra here, or we are sick of algebra, we can pick the numbers for a specific case to test the answer choices, though I believe the algebraic method will be faster if your algebra is up to speed. The probability of drawing a blue ball will be the number of blue balls over the total number of balls. After we add the balls, the total number of balls is . (E) leaps out as the only viable answer choice, and indeed the number of blue balls is .

The correct answer is (E).


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