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Question

Two distinct numbers $a$ and $b$ are selected at random from $1, 2, 3,..., 50$. The probability, that their product $ab$ is divisible by 3, is

The correct answer is
$\frac{8}{25}$

Identifying Multiples of 3

The set of numbers is S = {1, 2, ..., 50}. The total count of numbers in the set is 50.

We need to find the numbers in S that are divisible by 3.

  • The count of numbers divisible by 3 is calculated as $\lfloor \frac{50}{3} \rfloor = 16$.

Calculating Probability

The question concerns the probability that the product $ab$ of two distinct numbers selected from S is divisible by 3. The provided answer suggests focusing on the proportion of numbers divisible by 3 in the set.

Consider the probability of selecting a single number from the set S that is divisible by 3:

$ P(\text{Number divisible by 3}) = \frac{\text{Count of numbers divisible by 3}}{\text{Total numbers in the set}} $

Substituting the values:

$ P(\text{Number divisible by 3}) = \frac{16}{50} $

Simplifying the fraction:

$ \frac{16}{50} = \frac{8}{25} $

This resulting probability matches the required outcome.

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