The required natural numbers are
12,19,26,33,40,47,54,61,68,75,82,89,96
∴ Sum
=12+19+26+33+40+47+54 +61+68+75+82+89+96
=702 Alternate The required natural numbers are
12,19,26,...,96 Let there are
n terms in this series.
Then last term
=96=12+(n−1)7 ⇒,n−1==12 ⇒n=13 Now, sum of these terms
=13×54=702