A big contains 4 white balls and some red balls. If the probability of drawing a white ball from the bag is $\frac{2}{5}$ , find the number of red balls in the bag.

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Solution

The number of white balls is 4. Let the number of red balls is x. Then the total number of trials is.

Let A be the event of drawing a white ball.

The number of times A happens is 4.

Remember the empirical or experimental or observed frequency approach to probability.

If n be the total number of trials of an experiment and A is an event associated to it such that A happens in m-trials. Then the empirical probability of happening of event A is denoted byand is given by

Therefore, we have.

But, it is given that. So, we have

Hence the number of red balls is.

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