Adding and subtracting decimals

Subtract three point six eight from nine point two. Try it first. Pause if you like. Write nine point two as nine point two zero. Keep the decimal points directly under each other. Start at the hundredths. There are no hundredths to subtract eight from. Which place can we rename? Rename one tenth as ten hundredths. Two tenths become one tenth, and zero hundredths become ten hundredths. The value has not changed. Ten hundredths minus eight hundredths is two hundredths. Write two in the hundredths column. One tenth is not enough to subtract six tenths. Rename one whole as ten tenths. Nine ones become eight ones, and one tenth becomes eleven tenths. Eleven tenths minus six tenths is five tenths. Write five in the tenths column. Eight ones minus three ones is five ones. Keep the decimal point in its column. Nine point two minus three point six eight equals five point five two. Each renaming moved value into the next smaller place. It changed how the number was written, but not how much it was worth.

Find the value of six point four plus two point eight seven. Try it first. Pause if you like. Write six point four as six point four zero. The four stays in the tenths column, above the eight. After adding the hundredths, will the tenths need renaming? Zero hundredths plus seven hundredths is seven hundredths. Write seven. Four tenths plus eight tenths is twelve tenths. Rename ten of those tenths as one whole. Write two tenths and carry one to the ones column. Six ones plus two ones plus the carried one is nine ones. Write nine, with the decimal point aligned. Six point four plus two point eight seven equals nine point two seven. Match the decimal places. Rename without changing the value.