The sequence of differences between successive terms is 2, 6,18, 54...... Clearly, it is a GP. Let Tn be the n th term of the given series and Sn be the sum of its n terms. Then, Sn=5+7+13+31+85+....+Tn−1+Tn ....(i) Sn=5+7+13+31+....+Tn−1+Tn ......(ii) Subtracting (ii) from (i), we get O=5+[2+6+18+54+.....+(Tn−Tn−1)]−Tn ⇒ O=5+2.