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Among four numbers, the mean of the first three is 18 and the mean of the last three is 16. Given that the fourth (last) number is 19, what is the first number?

1. 19
2. 18
3. 20
4. 25

Correct answer: 25

Solution

Sum of first three = 54 and sum of last three = 48, so (first - last) = 54 - 48 = 6. With last = 19, first = 19 + 6 = 25.

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