It is given that,
Length (l) of room = 5 m
Breadth (b) of room = 4 m
Height (h) of room = 3 m
It can be observed that four walls and the ceiling of the room are to be whitewashed. The floor of the room is not to be whitewashed.
Area to be whitewashed = Area of walls + Area of ceiling of the room
= 2lh + 2bh + lb
=[2×5×3+2×4×3+5×4]m2=(30+24+20)m2=74m2
Cost of whitewashing per m2 area = Rs. 7.50
Cost of whitewashing 74 m2 area=Rs.(74×7.50)=Rs.555