Maths-
General
Easy

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 -

  1. 6    
  2. 7    
  3. 8    
  4. 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
    rightwards double arrowy = 20 – 3x.
    As 0 less or equal than y 20, we get 0 less or equal than 20 – 3x less or equal than 20
    rightwards double arrow 0 less or equal than 3x less or equal than 20 or 0 less or equal thanless or equal than 6
    thereforeThe number of ways of selecting the balls is 7.

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