Multiple choice

Out of 10 balls, some are red and others are white. The average cost of all the balls is Rs. 28. If the average cost of a red ball is Rs. 25 and that of a white ball is Rs. 30, then find the number of white balls.

  1. 3

  2. 5

  3. 6

  4. 7
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let R be red balls, W be white balls. R+W=10. Total cost = 28 * 10 = 280. 25R + 30W = 280. Substitute R = 10-W: 25(10-W) + 30W = 280. 250 - 25W + 30W = 280. 5W = 30. W = 6.

AI explanation

The total cost of all 10 balls is 10 multiplied by 28, which equals 280 rupees. Let the number of white balls be w; then the number of red balls is 10 minus w. Using the allegation method, the ratio of red balls to white balls is (30 minus 28) to (28 minus 25), which is 2 to 3. Since the total is 10 balls in a 2 to 3 ratio, the number of white balls is 3 fifths of 10. The result is 6 white balls.

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