Question

# There are three piles of identical yellow, black and green balls and each pile contains at least 20 balls. The number of ways of selecting 20 balls if the number of black balls to be selected is twice the number of yellow balls, is -

- 6
- 7
- 8
- 9

## The correct answer is: 7

### Let the number of yellow balls be x, that of black be 2x and that of green be y. Then

x + 2x + y = 20 or 3x + y = 20

y = 20 – 3x.

As 0 y 20, we get 0 20 – 3x 20

0 3x 20 or 0 x 6

The number of ways of selecting the balls is 7.

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