Explanation

This is a weighted arithmetic mean problem: with weights wᵢ equal to the numbers themselves (wᵢ = i), the mean is Σwᵢxᵢ/Σwᵢ = Σi·i/Σi = Σi²/Σi. The standard sums are Σi² = 10·11·21/6 = 385 and Σi = 10·11/2 = 55, so the weighted mean is 385/55 = 7. Hence option (a). The distractors come from confusing the weighted mean with the ordinary mean (5.5) or with the raw sums 35 and 38.5.