A room is 15 m long and 8 m wide. It is to be covered with a carpet 75 cm wide. What is the cost of the carpet at ₹90 per metre of its length?

The floor is $15\times 8=120$ sq m. A carpet 0.75 m wide must run a length of 120/0.75 = 160 m to cover it, so the cost = $160\times 90$ = ₹14400. The trap ₹10800 applies the ₹90 rate to the 120 sq m of area, but the rate is per metre of LENGTH, not per square metre; ₹8100 multiplies by 0.75 instead of dividing; ₹144 reads the width as 75 m.