Multiple choice

The average age of 30 students is 20 years. When the teacher's age is included, the average becomes 21 years. The teacher's age is:

  1. 41

  2. 50

  3. 51

  4. 60
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Total balls = 4 + 5 + 6 = 15. Total ways to choose 3 balls is 15C3 = (15*14*13)/(3*2*1) = 455. Ways to choose 3 red = 4C3 = 4; 3 blue = 5C3 = 10; 3 green = 6C3 = 20. Total favorable = 4 + 10 + 20 = 34. Probability = 34/455.

AI explanation

The total age of the 30 students is 30 times 20, which equals 600 years. When the teacher is included, the total age becomes 31 times 21, giving 651 years. The teacher's age is the difference between the new total and the old total, which is 651 minus 600, resulting in 51 years.

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