There are in all, 10 balls; some of them are red and the

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There are in all, 10 balls; some of them are red and the others white. The average cost of all balls is ₹ 28. If the average cost of red balls is ₹25 and that of white balls is ₹30, the number of white balls is :

1. 3
2. 5
3. 6
4. 7

Correct Option: C

Here , The average cost of all balls = ₹ 28.
The average cost of red balls = ₹25
and the average cost of white balls = ₹30
Let the number of white balls be y.
∴ Number of red balls = (10 – y)
∴ 10 × 28 = y × 30 + 25 (10 – y)
⇒ 280 = 30y + 250 – 25y = 5y + 250
⇒ 5y = 280 – 250 = 30
⇒ y = 6