Question

A bag contains 20 red balls and some green balls. The probability of drawing a green ball from the bag is twice that of drawing a red ball. Find the number of green balls in the bag.

Answer 1

$\frac{20}{x} \times 2 = \frac{40}{x} \\ \\\frac{40}{x} + \frac{20}{x} = 1 \\ \\ \frac{40 + 20}{ x} = 1 \\ \\ \frac{60}{x} = 1 \\ \\ x = 60 \\ \\ no. \: of \: green \: balls = \\ \\ 60 - 20 = 40$

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Answer 2

Given : A bag contains 20 red balls and some green balls. The probability of drawing a green ball from the bag is twice that of drawing a red ball.

To find : The number of green balls in the bag.

solution : let the number of green balls in the bag is x.

so total number of balls in bag, n(s) = x + 20

now probability of red ball, P(red) = n(red)/n(S)

= 20/(x + 20)

probability of green ball, P(green) = n(green)/n(S)

= x/(x + 20)

a/c to question,

P(green) = 2 × P(red)

⇒ x/(x + 20) = 2 × 20/(x + 20)

⇒x = 40

Therefore number of green balls in the bag is 40.