মান নির্ণয় করুন: 6 + 11 + 16 + 21 + … + 71 A. 539 B. 561 C. 661 D. 639

This is an arithmetic series with first term 6, common difference 5 and last term 71, so it has (71 − 6)/5 + 1 = 14 terms. Sum = 14 × (6 + 71)/2 = 14 × 38.5 = 539.