Answer:
20 cm = 0.2 m
10 cm = 0.1 m
5 cm = 0.05 m
1 km = 1000 m
5 m = 5 m
50 cm = 0.5 m
The volume of each brick is:
V_brick = 0.2 m x 0.1 m x 0.05 m = 0.001 m^3
The volume of the wall is:
V_wall = 1000 m x 5 m x 0.5 m = 2500 m^3
The number of bricks required is:
N_bricks = V_wall / V_brick = 2500 m^3 / 0.001 m^3 = 2,500,000
Therefore, 2,500,000 bricks of size 20 cm * 10 cm * 5 cm will be required to construct the wall.