Breadth of the room is 75% of 8m=6m, so the area is 48 sq m.
The roll of carpet is assumed to be 1m wide, so 48m of the carpet would have an area of 48 sq m. Therefore the carpeting would require 6 strips of carpet, each 8m long. The cost of each strip is 8*39=Rs 312 and 6 strips would cost 6*312=Rs 1872.
If the roll of carpet was 2m wide, then only 3 strips would be required with a cost of 3*312=Rs 936.