The average of 25 results is 18. The average of the first 12 is 14 and the average of the last 12 results is 17. Find the 13th result.

Correct option is B

Ans. (b)
Sol.
Let the 13th result be x.
Step 1: Total sum of all 25 results = 25 × 18 = 450
Step 2: Sum of first 12 results = 12 × 14 = 168
Step 3: Sum of last 12 results = 12 × 17 = 204
Now,
Sum of all 25 results = (Sum of first 12) + (Sum of last 12) + 13th result
⇒ 450 = 168 + 204 + x
⇒ 450 = 372 + x
⇒ x = 450 – 372 = 78
So, the 13th result is 78.