Fifth Grade · Math · week 4

Add decimals: line them up

Add decimals with up to thousandths by aligning place value, including numbers with different numbers of decimal places.

Adding decimals works like adding whole numbers, but you must line up the decimal points first -- not the digits themselves. That keeps tenths under tenths and hundredths under hundredths.

If one number has fewer decimal places, fill the empty spots with zeros. A placeholder zero never changes the value, and it keeps every column lined up correctly.

📋 For the teacher
Teach it (10–15 min):
  • Have them rewrite both numbers stacked with decimal points directly under each other before adding a single digit.
  • Say the placeholder zeros out loud: "3.45 plus 2.7 becomes 3.45 plus 2.70."
  • Bring the decimal point straight down into the answer -- it never shifts during addition.
  • Estimate first (3 plus 3 is about 6) so a misaligned column gets caught before it becomes the final answer.
Talk about it:
  • Why must decimal points line up before you add?
  • What does writing a placeholder zero do to the value of a number?
  • How can estimating first catch a mistake?

Mastery looks like: They can add multi-digit decimals of different lengths by aligning decimal points and regrouping correctly.

Common stumbles: Lining up the rightmost digits instead of the decimal points; forgetting to bring the decimal point straight down into the answer.

See it visually

Interactive, spoken-aloud tools that show this idea in motion.

AdditionSee groups combine — with counters, blocks, and the number line.try →Place ValueWatch ones, tens, hundreds, and thousands build big numbers.try →
Homework for this lessonType it, write it with a stylus, or print it for pencil & paper.open →
Records attendance + 15 min of Math for this learner.
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