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

Total Area to be washed = lb + 2(l + b)h

Where, length (l) = 5 m

breadth (b) = 4 m

height (h) = 3 m

Therefore, the total area to be white washed is = (5*4) + 2*(5 + 4)*3

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

Now, The cost of white washing $1 \mathrm{~m}^{2}$ is Rs. $7.50$

Therefore, the cost of white washing $74 \mathrm{~m}^{2}=\left(74^{*} 7.50\right)$

= Rs. 555/-