The sale of in the world of reality, using a math help, such as in the Exchange, such as you have the money to buy 30 baht per baht 20 bar pens to get how much change will need to take math to help 30-20 to change 10/ .
The sale of the real world using mathematics to help, such In exchange, you are paid 30 baht to 20 baht each pen must be change, it will require 30-20 mathematics helped change will be 10 baht.
Sales of the real world by using mathematics to help such as in exchange, such as you have 30 BA ทซื้อ pen? 20) will have to be much change to use mathematics to help 30-20 will get change 10บาท .