11, 13 …… 29, 31Nth term of series = 2n + 5For total number of terms,2n + 5 = 31⇒ n = 137 + 9 + 11 + 13 + …… 31= \(\mathop \SUM \limits_1^{13} \left( {2n + 7} \right)\; = 2 \times \mathop \sum \limits_1^{13} n + 5 \times \mathop \sum \limits_1^{13} 1\)\(\mathop \sum \limits_1^n n = \frac{{n\left( {n + 1} \right)}}{2}\)\(\mathop \sum \limits_1^n 1 = n\)⇒ 7 + 9 + 11 + 13 + …… 31 = 2 × (13 × 14)/2 + 5 × 13 = 247