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.