The length, breadth and height of a room are 5 m, 4 m and 3 m respectively.

Solution:

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 white-washed. The floor of the room is not to be white-washed.

Area to be white-washed = Area of walls + Area of ceiling of room

$=2 l h+2 b h+l b$

$=[2 \times 5 \times 3+2 \times 4 \times 3+5 \times 4] \mathrm{m}^{2}$

$=(30+24+20) \mathrm{m}^{2}$

$=74 \mathrm{~m}^{2}$

Cost of white-washing per $\mathrm{m}^{2}$ area $=$ Rs $7.50$

Cost of white-washing $74 \mathrm{~m}^{2}$ area $=\mathrm{Rs}(74 \times 7.50)$

$=$ Rs 555