Maryland Math 5.NOS.B.6.a

Math5th GradeNumber and Operation Sense

The Standard

5.NOS.B.6: Apply and extend previous understanding of addition and subtraction of whole numbers to add and subtract decimals to hundredths.

Apply estimation strategies to estimate sums and differences of decimals.

Maryland College and Career Ready Standards for Mathematics (2025) · PERFORM OPERATIONS ON MULTI-DIGIT WHOLE NUMBERS AND WITH DECIMALS TO HUNDREDTHS.

Teacher's field guide

What Students Need to Do: Students estimate decimal sums and differences by rounding or using nearby numbers that are easy to work with. They choose a place value that gives a useful estimate.
What Mastery Looks Like: A student can estimate 6.78 + 2.19 as about 9 and 8.49 - 8.41 as about 0.1. They can explain why rounding to whole numbers works for the first problem but not the second.
Common Misconceptions: Students may round 4.78 to 4 by dropping the decimal digits. They may round two close numbers to whole numbers, get a difference of zero, and miss a small difference.
How to Assess It: Exit ticket: Estimate 12.08 - 5.91 and 8.49 - 8.41 without finding exact answers. Show the rounded numbers you used and explain why you chose different place values.

Lesson moves

Give pairs digit cards and place-value mats to build 8.49 and 8.41, then estimate their difference using tenths.
Ask: Why does rounding 8.49 and 8.41 to whole numbers give an unhelpful difference? Write a better estimate.
Play Estimate Match: students pair decimal problem cards with rounded-number cards and estimate cards, then explain one match.
Show a receipt with items costing $6.78 and $2.19; ask students to estimate whether $10 will cover both.

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