here ...
let us suppose breadth of the room be x.
then, length = 2x
now ,the cost of plastering the floor is RS 1080 at rate of RS 15 per ...
hence
the area of floor{length *breadth} =1080/15
x*2x=72
or,2x^2=72
or,x=√(72/2)
hence, the value of x= 6m
(ie. breadth {x}=6. &. length {2x}=12)
again....
we have ,
volume of the room (length *breadth*height)is 396m^3
12*6*height=396
72*height=396
height=396/72
hence , height=5.5
we know,
the area of 4 walls = 2( length+breadth)*height
=2(12+6)*5.5
=198
hence the cost of plastering 4 walls at the rate of RS 20 per SQ meter = 198*20=RS 3960