Multiple choice

Out of 5 numbers the average of first 4 numbers is 20 and the last four numbers is 18. If the last number is 15, the first number is

  1. 20

  2. 22

  3. 21

  4. 23
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Let the numbers be a, b, c, d, e. (a+b+c+d)/4 = 20 => a+b+c+d = 80. (b+c+d+e)/4 = 18 => b+c+d+e = 72. Given e=15, then b+c+d = 72-15 = 57. Substituting into the first equation: a + 57 = 80, so a = 23.

AI explanation

The first number equals the sum of the first four numbers minus the sum of the last four numbers, plus the last number. The sum of the first four numbers is 4 times 20, which is 80, and the sum of the last four numbers is 4 times 18, which is 72. Adding the last number to their difference gives 80 minus 72 plus 15, which equals 23. The first number is 23.

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