A square water tank has its side equal to 40 m.

It is given that the side of square $a=40 \mathrm{~m}$ -

Since four semicircular grassy plots rounds a square water tank. Then, diameter of semicircular plot is $2 r=a$.

So, the radius of semicircle

$r=\frac{a}{2}$

$=\frac{40}{2}$

$=20 \mathrm{~m}$

Area of semicircular plot $=\frac{1}{2} \pi r^{2}$

$=\frac{1}{2} \times 3.14 \times 20 \times 20$

$=628 \mathrm{~m}^{2}$

Now, the total area of plot is sum of area of four semicircular plots.

Total Area of plot $=4 \times$ Area of semicircle

$=4 \times 628 \mathrm{~m}^{2}$

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

Since, The cost of turfing the plot per square meter $=$ Rs $1.25$

So, The cost of turfing 2512 square meter plot $=$ Rs $1.25 \times 2512$

$=\operatorname{Rs} 3140 /-$