The mean of twelve positive numbers is 3. When another number is added, the mean becomes 5. Find the thirteenth number

  • A 29
  • B 26
  • C 25
  • D 24
jamb 1994mathematics

The correct answer is A. 29

Let’s say the sum of the first twelve positive numbers is S. Since the mean of these twelve numbers is 3, we have S/12 = 3, so S = 36. 

When a thirteenth number, say x, is added, the new mean becomes 5. So, (S + x)/13 = 5.

Substituting the value of S we found earlier, we have (36 + x)/13 = 5. Solving for x, we find that x = 65 - 36 = 29. 

So the thirteenth number is 29.

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