Number and Operations in Base Ten
Adding and subtracting decimals
Add and subtract numbers with tenths and hundredths by keeping the columns lined up.
Adding decimals works exactly like adding whole numbers, with one extra care: tenths must be added to tenths and hundredths to hundredths. Get the columns lined up and the sum looks after itself.
Watch out
0.5 and 0.50 are the same number. Writing the extra zero changes nothing about its value, and it makes the columns match — which is why it is worth writing.
Worked example
What is 3.2 + 1.45?
Write 3.2 as 3.20 so both numbers have hundredths.
Two tenths is twenty hundredths. The number has not changed, only how it is written.
Try it together
Let us work out 4.6 + 2.35 together.
Same plan: even up the decimal places first, then add column by column.
1.Rewrite 4.6 so that it shows hundredths as well as tenths.
Have a play
This line runs from 0 to 1. The numbered ticks are tenths, and the plain ticks between them fall halfway again. Look at how far apart 0.05 and 0.5 really are.
Tap anywhere on the line.
Have a go
Have a go on your own: 5.4 + 1.25 = ?
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.