Performing addition and subtraction with decimal numbers requires strict adherence to positional place-value alignment and disciplined computational execution. The cardinal rule of decimal computation is aligning the decimal points vertically, which ensures that digits occupying identical place values—such as tenths with tenths and hundredths with hundredths—are added or subtracted together. When addends or minuends possess differing decimal lengths, annexing placeholder zeros prevents catastrophic errors during regrouping. In addition to standard pencil-and-paper algorithms, students employ diverse mental estimation strategies, including front-end estimation, rounding estimation, and compatible numbers. These estimation techniques serve as essential sanity checks, enabling students to verify the mathematical reasonableness of their exact computed answers.