Multiple choice

Out of the red and white balls in a packet, the possibility of selecting a red ball is $\frac { a }{ b } $. Write the probability of selecting a white ball in terms of $a$ and $b$.

  1. $\dfrac {a - b}{b}$
  2. $\dfrac {b - a}{ b}$
  3. $\dfrac {a}{b}$
  4. $\dfrac {b}{a}$
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B Correct answer
Explanation

Total probability is 1. If P(red) = a/b, then P(white) = 1 - a/b = (b-a)/b.

AI explanation

According to the sum of probabilities rule, the probability of an event occurring plus the probability of its complementary event not occurring equals 1. Since the probability of selecting a red ball is given as the fraction a/b, the probability of selecting a white ball is 1 - (a/b). To subtract these fractions using a common denominator, the result is (b - a) / b.

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