Adding and Subtracting Decimals

The structure of decimals, down to the tenths, hundredths and thousandths place, is grasped through Place Value and Definition, Comparing Sizes and Number Line. Addition and subtraction follow in column and horizontal form, extending to sums and differences of three numbers and to Removing Trailing Zeros.

Where the two numbers have different numbers of decimal places, the standard error in subtraction is copying the digit above straight down rather than filling the empty place with 0. An answer of 2.44 for 3.6 - 1.24 means the 4 in the hundredths place was copied down. Finishing without writing the decimal point, as though the answer were a whole number, also persists.

The number of decimal places in play widens sharply here. Decimals in grade 3 stopped at a single carry, but this unit reaches the thousandths place and pairs numbers with different digit counts against each other. Lining up the decimal point to add or subtract carries forward as the key to placing it correctly once Multiplying and Dividing Decimals begins.