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Out of four numbers, the average of the first three is 16 and that of the last three is 15. If the last number is 20, the first number is:

1. 23
2. 25
3. 28
4. 21

Correct answer: 23

Solution

Let the numbers be a, b, c, d. Then a+b+c = 48 and b+c+d = 45. Since d = 20, we get b+c = 25, so a = 48 - 25 = 23.

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