Please help me with the following question

Dear Student, 

the area of the four walls = 2*(6+4)*3 = 2*30 sq m. = 60 m²
Area to be papered=(60-11)=49 m² (Area of door is excluded)

the area of each piece = 0.4 *0.6 sq m = 0.24 sq m

therefore the number of pieces of paper required = 49 / 0.24 = 204.17

i.e. 205 (approx)

the cost of paper = Rs. 1.4*205 = Rs. 287

Regards!