Explanation:

speed of the man = 4 x (5/18) m/sec = 10/9 m/sec
time taken = 3 x 60 sec = 180 sec
length of diagonal = speed x  time = (10/9) x 180 = 200m
Area of the field = (1/2) x (dioagonal)²
= (1/2) x 200 x 200 sq.m = 20000sq.m