Calculate the area of the four walls

The lateral surface area of a cuboid (walls only) is given by the formula $2h(l + b)$. Plugging in the values: $2 \times 21 \times (15 + 17) = 42 \times 32 = 1344$ m².

$Area_{walls} = 2 \times 21 \times (15 + 17) = 1344 \text{ m}^2$

Calculate the area of the ceiling

The area of the ceiling is the same as the floor area, given by $l \times b$. Plugging in the values: $15 \times 17 = 255$ m².

$Area_{ceiling} = 15 \times 17 = 255 \text{ m}^2$

Calculate the total area to be painted

Add the wall area and the ceiling area: $1344 + 255 = 1599$ m².

$\text{Total Area} = 1344 + 255 = 1599 \text{ m}^2$

Calculate the total cost

Multiply the total area by the rate of painting: $1599 \times 42$. Calculation: $1599 \times 42 = 67158$.

$\text{Total Cost} = 1599 \times 42 = \text{₹}67158$