Question

There are 10 pens in a box. There are x red pens in the box. All the other pens are blue. Jack takes at random two pens from the box. Find an expression, in terms of x, for the probability that Jack takes one pen of each colour. Give your answer in its simplest form.​

Answer 1

The probability that one of each color is selected is $\frac{10x - x^2}{45}$

Probabilities

The probability of an event is the chances of the said event

The given parameters are:

• Total  = 10
• Red = x
• Blue = 10 - x

Calculating the required probability

The probability that one of each color is selected is calculated as follows:

$P = P(Blue) \times P(Red) + P(Red) \times P(Blue)$

So, we have:

$P = \frac{10 - x}{10} \times \frac{x}{9} + \frac{x}{10} \times \frac{10 - x}{9}$

This gives

$P = \frac{x(10 - x)}{90} +\frac{x(10 - x)}{90}$

Take LCM

$P = \frac{2x(10 - x)}{90}$

Simplify the above expression

$P = \frac{x(10 - x)}{45}$

Expand

$P = \frac{10x - x^2}{45}$

Hence, the probability that one of each color is selected is $P = \frac{10x - x^2}{45}$

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