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

Given, sum of 13 results 70 × 13 = 910

Sum of the first 7 results = 7 × 65 = 455

And sum of the last 7 results = 7 × 75 = 525

        Seventh observation

=         Sum of the first 7 results

 +      Sum of last 7 results

 -        Sum of ‘13’ results

= 455 + 525 - 910

= 980 - 910

= 70

 So, option ‘B’ 70 is correct answer.