The perimeter of a floor of a room is 30 m and its height is 3 m.

Perimeter of the floor of the room $=30 \mathrm{~m}$

Height of the oom $=3 \mathrm{~m}$

Perimeter of a rectangle $=2 \times($ length $+$ breadth $)=30 \mathrm{~m}$

So, area of the four walls $=2 \times($ length $\times$ height $+$ breadth $\times$ height $)$

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

$=30 \times 3=90 \mathrm{~m}^{2}$