Peter built a room in his garden to write stories and to spend time alone. She wants to spread the carpet in his room. He went to the store to buy the carpet. The shopkeeper told him that the price per square foot of carpet was $ 10.
The shopkeeper said" how much carpet do you need".
Peter replied "I know the size of the outside of the room. Not measured inside. The outside of the room is 20 feet long and 10 feet wide. The width of the wall is 1 foot."

The shopkeeper took a notebook and a pen and started calculating the area inside the room.

The shopkeeper first drew such a figure

The shopkeeper wrote some information

Room length = 20 feet
Room width = 10 feet
Room wall width = 1 foot

Then he began to calculate

Since the width of the wall is 1 foot
Then the length inside the room = 20- (1 + 1) = 18 feet

Peter then asked the shopkeeper you should subtracted 1 but why you subtracted 2.
Then the shopkeeper replied that there are walls on both sides of the length of the room so 2 will be subtracted

The shopkeeper wrote again

The width inside Room = 10- (1 + 1) = 8 feet

Floor area inside the room = 18 * 8 =144 square feet

The shopkeeper said you need 144 square feet of carpet. your carpet price is (144*10)=1440 USD

Peter understood the calculation. He bought the carpet and took it home. When he buys carpets again, he no longer needs the help of shopkeepers to calculate the floor area of ​​the room.