# Ex.15.2 Q3 Probability Solution - NCERT Maths Class 10

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## Question

A bag contains $$5$$ red balls and some blue balls. If the probability of drawing a blue ball is double that of a red ball, determine the number of blue balls in the bag.

## Text Solution

What is known?

A bag contains $$5$$ red balls and some blue balls and the probability of drawing a blue ball is double that of a red ball.

What is unknown?

The number of blue balls in the bag.

Reasoning:

First find out the total number of outcomes and all the possible outcomes. Now, to find the probability use the formula given below

\begin{align}\text{ Probability }=\frac{\text{No of possible outcomes}}{\text{ Total no of outcomes }} \\\end{align}

As it is given in the question that the probability of drawing a blue ball is double that of a red ball, that means

$$2 \times$$probability of drawing red ball = probability of drawing blue ball.

Substitute the values in the above and you can find out the number of blue balls.

Steps:

No of red balls $$= 5$$
Let the no of blue balls be  $$x$$
Total number of balls $$= x+5$$

Probability of drawing red ball

\begin{align}&= \frac{\text{ No of possible outcomes }}{\text{ Total no of outcomes }} \\&=\frac{5}{x+5}\end{align}

Probability of drawing blue ball

\begin{align}&= \frac{\text{ No of possible outcomes }}{\text{ Total no of outcomes }} \\&=\frac{x}{x+5}\end{align}

Thus, the probability of drawing a blue ball is double that of a red ball

\begin{align}2\left( \frac{5}{x+5} \right)=\frac{x}{x+5}\end{align}

Hence the number of blue balls $$= 10$$

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