A square tank has an area of 1600 cm2.

Let a m be the side of the square.

Area of the square $=a^{2}$

Thus, we have:

$a^{2}=1600$

$\Rightarrow a=40$

Area of the plots = 4(Area of the semicircle of radius 20 m)

$=\left|4\left(\frac{1}{2} \pi r^{2}\right)\right| \mathrm{m}^{2}$

$=\left|4\left(\frac{1}{2} \times 3.14 \times 20 \times 20\right)\right| \mathrm{m}^{2}$

$=2512 \mathrm{~m}^{2}$

$\therefore$ Cost of turfing the plots at $12.50$ per $\mathrm{m}^{2}=\mathrm{Rs}(2512 \times 12.50)$

$=$ Rs 31400