Let the Height be of value X.

Therefore ,  Length = 3X.

                      Breadth = 2X.

Height is a common factor to all the 4 walls.

So Total Area of 4 walls (Initially) = 10 X ^ 2

Next ,

Breadth = X,

Length = 6 X

Height = X/2

So total Area now = 7 X ^2

Therefore , the total area decreases by 30 %

So Answer is 5

Hope U have got it clear.

Cheers