The length, breadth and height of a room are 5 m, 4 m and 3 m respectively. find the cost of white-washing the walls of the room and ceiling at the rate of $ 7.50 per m^2.

Given that,
the length of the room (l) = 5 m
breadth of the room (b) = 4 m
and height of the room (h) = 3 m

Therefore, total surface area to be white washed
= Area of the four wallls + Area of the ceiling
= 2(l + b) × h + l × b
=2 ( 5+4) × 3 m² + 5 × 4 m²
= 2 × 9 × 3 m² +20 m²
= (54 +20) m² =74 m²

Since, cost of white washing 1 m² = $ 7.50
Therefore, cost of white washing 74 m²= $ 7.50 ×74 = $555

Hence, the cost of white washing the walks and ceiling of the room is $ 555.