A classroom is 7 m long, 6 m broad and 3.5 m high.

Length of the classroom $=7 \mathrm{~m}$

Breadth of the classroom $=6 \mathrm{~m}$

Height of the classroom $=3.5 \mathrm{~m}$

Total surface area of the classroom to be whitewashed $=$ areas of the 4 walls

$=2 \times($ breadth $\times$ height $+$ length $\times$ height $)$

$=2 \times(6 \times 3.5+7 \times 3.5)$

$=2 \times(21+24.5)$

$=91 \mathrm{~m}^{2}$

Also, the doors and windows occupy $17 \mathrm{~m}^{2}$.

So, the remaining area to be whitewashed $=91-17=74 \mathrm{~m}^{2}$

Given that the cost of whitewashing $1 \mathrm{~m}^{2}$ of wall $=$ Rs $1.50$

$\therefore$ Total cost of whitewashing $74 \mathrm{~m}^{2}$ of area $=74 \times 1.50=$ Rs 111