A man walking at the speed of 4 km/hr,cross a square field diagonally in 3 minutes.The area of the field is?

Speed of man,
= 4 kmph = $$\frac{{4 \times 5}}{{18}}$$ m/sec
In 3 min (180 sec) man will go = $$\frac{{20 \times 180}}{8}$$ = 200 m.
That means the diagonal of the square field = 200 m.
Diagonal of square,
= Side of Square × $$\sqrt 2 $$  = 200 m.
→ Side of Square = $$\frac{{200}}{{\sqrt 2 }}$$
Area of Square = Side²
Area = $$\frac{{200 \times 200}}{2}$$   = 20000 sqm.