Correct Option: B

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 the last 7 results – Sum of 13 results
Seventh observation = 455 + 525 – 910
Seventh observation = 980 – 910 = 70
Second method to find the required result ,
Here, n = 13, x = 70 , y = 65 m = 7, z = 75
∴ Seventh result = m (y + z) – nx
Seventh result = 7 (65 + 75) – 13 × 70
Seventh result = 7 × (140) – 910
Seventh result = 980 – 910 = 70