Multiple choice

Out of four numbers, the average of first three is 15 and that of the last three is 16. If the last number is 19, the first number is

  1. 15

  2. 16

  3. 18

  4. 19
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B Correct answer
Explanation

Let the numbers be a, b, c, d. (a+b+c)/3 = 15 => a+b+c = 45. (b+c+d)/3 = 16 => b+c+d = 48. Given d = 19, then b+c = 48 - 19 = 29. Substituting into the first equation: a + 29 = 45, so a = 16.

AI explanation

The sum of the first three numbers is 15 times 3, equaling 45. The sum of the last three numbers is 16 times 3, equaling 48. The difference between these sums is 48 minus 45, which equals 3, meaning the third number subtracted from the fourth number is 3; since the last number is 19, the first number is 19 minus 3, resulting in 16.

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