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The average of 13 results is 70. The average of the first seven is 65 and that of the last seven is 75. The seventh result is:

1. 67
2. 70
3. 68
4. 70.5

Correct answer: 70

Solution

Total of 13 results = 13 × 70 = 910. Sum of first seven = 7 × 65 = 455 and sum of last seven = 7 × 75 = 525. Since the 7th result is counted twice in these two sums, it equals 455 + 525 - 910 = 70.

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