The document discusses addition and subtraction. It defines addition as combining two quantities A and B to obtain a sum S, where A and B are the addends. To add two numbers, one lines them up vertically according to place value and adds the digits from right to left, carrying when necessary. Subtraction is defined as taking away or undoing addition. To subtract, one lines numbers up vertically and subtracts the digits from right to left, borrowing when needed. Real-life word problems involving addition and subtraction are translated into mathematical expressions by clarifying which quantity is being added to or subtracted from the other.