Add and Subtract Decimals
Lessons in this chapter
About Add and Subtract Decimals
Decimals and their Addition and Subtraction extends decimals to a third place and makes comparing and rounding them routine.
Read and Write Decimals to Thousandths adds the third decimal place. Each place continues to be a tenth of the one before, so the structure is unchanged, but reading 0.407 correctly as four hundred seven thousandths takes care because of the zero.
Compare and Order Decimals to Thousandths is where the persistent error lives. Kids read 0.45 as larger than 0.5 by comparing 45 with 5, and the fix is to compare place by place from the left, exactly as with whole numbers, or to pad with zeros so both have the same number of places.
Round Decimals applies the same rule used for whole numbers: look at the digit to the right of the target place. Add and Subtract Decimals needs only the decimal points lined up, which is place value alignment with a visible marker.
All four lessons open as a 5th grade math worksheet in the page. Compare and Order Decimals to Thousandths, Round Decimals and Add and Subtract Decimals each set their own questions, worked one at a time and marked as you answer them. They are free to try, with no login needed for your first 3.
Frequently asked questions
Read and Write Decimals to Thousandths adds the third decimal place, where the zero in 0.407 is what makes reading it aloud take care. Compare and Order Decimals to Thousandths is where the persistent error lives, since kids read 0.45 as larger than 0.5, and Round Decimals applies the same rule used for whole numbers. Add and Subtract Decimals closes the chapter and needs only the decimal points lined up. It sits in the Common Core domain Number and Operations in Base Ten (5.NBT).
Compare place by place from the left. Both have 0 ones; then 4 tenths against 5 tenths, so 0.5 is larger. Padding 0.5 to 0.50 makes it obvious.
Thousandths. After tenths and hundredths, each place continues to be a tenth of the one before it.
No. 0.5 and 0.50 are the same amount, because five tenths equals fifty hundredths. This is the opposite of whole numbers, where adding a zero multiplies by ten.
Look at the digit immediately to the right of the place you are rounding to. 5 or more rounds up, less rounds down. It is the same rule as for whole numbers.
Line up the decimal points. Padding the shorter number with zeros keeps the columns straight and prevents the usual misalignment errors.