Length of the room = 4 m = 400 cm
Breadth of the room = 3 m = 300 cm
Area of the floor of the room = Length × Breadth
= 400 × 300 sq cm = 120000 sq cm
Side of the square tile = 20 cm
Area of the square tile = Side × Side = 20 × 20 sq cm
= 400 sq cm
So, number of tiles required = Area of the floor/Area of one tile = 120000/400 = 300