Question 850062
Total area to paint of the room ignoring windows or door:  {{{3*3*5*m^2}}}.
The description already specified, "square room"; all edges of this room are 3 meters.

Amount of paint to use:  {{{3*3*5*m^2(1*liter/(16*m^2))=(45/16)*liters}}};
That was probably not what you wanted.


Maybe you are trying this:
The room is square around the floor, but this room is 3 meters high.  
Let d = side length of the square shaped floor of this room.  The total surface to be painted, the walls and the cieling, is  {{{4*3*d+d^2=d^2+12d}}}.
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The 10 liters of paint used means that, according to usage ratio, the area covered was {{{10*liter(16*m^2)/(1*liter)=160*m^2}}} of surface.


Those two surface areas must be equal.
{{{highlight_green(d^2+12d=160)}}}.
Solve for d, which is as much the room length as it is the room width.  (Remember, square room, at least for the floor and the cieling).
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160=2*5*4*4=20*8
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{{{d^2+12d-160=0}}}
{{{(d+20)(d-8)=0}}}
{{{highlight(highlight(d=8))}}}------- Room length or width must be 8 meters.