Multiple choice

There are 20 balls, some of them are golden and the others are yellow. The average cost of all balls is Rs. 38, average cost of golden balls is Rs. 35 and that of yellow balls is Rs. 40, find the number of yellow balls.

  1. 15

  2. 18

  3. 12

  4. 16
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let yellow balls = y, golden balls = 20 - y. Total cost = 20 × 38 = 760. Golden cost: 35(20 - y), Yellow cost: 40y. Total: 35(20 - y) + 40y = 760. Solving: 700 - 35y + 40y = 760, 5y = 60, y = 12. Using allegation method: (40 - 38):(38 - 35) = 2:3, so yellow:golden = 2:3, giving yellow = 2/5 × 20 = 12.

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