Soldiers in concentric circles

Let the three concentric circles be $C_1, C_2, C_3$ with radii $r_1, r_2, r_3$. Given $r_1 = 70\text{ m}$ and $r_3 = 140\text{ m}$, and since $r_1, r_2, r_3$ are in AP, $r_2 = \frac{70 + 140}{2} = 105\text{ m}$.

The total perimeter of all three circles is: $\text{Total Circumference} = 2\pi(r_1 + r_2 + r_3) = 2 \times \frac{22}{7} \times (70 + 105 + 140) = 2 \times \frac{22}{7} \times 315 = 1980\text{ m}$

Since soldiers are separated by $1\text{ m}$ on each circumference, the number of soldiers required is $1980$.