Adding Decimals

To add decimals, line up the decimal points so that tenths sit over tenths and hundredths over hundredths. Take 3.8 + 1.45. Write 3.8 as 3.80 so both numbers have two places, then add column by column, just as with whole numbers:

3.80 + 1.45 = 5.25

The hundredths give 0 + 5 = 5. The tenths give 8 + 4 = 12: write the 2 and carry 1 into the ones, which then give 3 + 1 + 1 = 5. The sum is 5.25.

Why line up the points? You can only add like with like, tenths with tenths, just as you add tens with tens in a whole number. Lining up the last digits instead would add 8 tenths to 5 hundredths, two different sizes of piece, like adding 8 tens to 5 ones.