Maryland Math 5.NOS.C.8.a
The Standard
- 5.NOS.C.8
- Add and subtract fractions with unlike denominators.
Estimate sums and differences by reasoning about benchmark numbers and assess reasonableness of answers (e.g., 2/3 + 3/4 will be more than 1 because both fractions are greater than 1/2).
Maryland College and Career Ready Standards for Mathematics (2025) · USE EQUIVALENT FRACTIONS AS A STRATEGY TO ADD AND SUBTRACT FRACTIONS.
Teacher's field guide
What This Standard Means
What Students Need to Do
- Students use 0, 1/2, and 1 to estimate sums and differences of fractions. They use those estimates to check whether an answer makes sense.
What Mastery Looks Like
- For 3/4 + 2/3, a student predicts a sum near 1 1/2 and rejects 5/7. For 7/8 - 2/5, they expect about 1/2 and reject an answer greater than 1.
Common Misconceptions
- Some students add numerators and denominators to get 5/7 for 2/3 + 3/4, even though both fractions exceed 1/2. Others treat a benchmark estimate as an exact answer or accept a difference larger than the starting fraction.
How to Assess It
- Exit ticket: Estimate 2/3 + 3/4 and 7/8 - 2/5 using 0, 1/2, or 1. Explain why 5/7 and 1 1/4 are not reasonable answers, respectively.
Lesson moves
Ways to Teach It
Place 2/3 and 3/4 fraction strips end to end beside a whole strip, then decide whether their sum is nearer 1 or 1 1/2.
Write two sentences explaining why 7/8 - 2/5 is near 1/2, using benchmark numbers.
Play 'Reasonable or Not' with 2/3 + 3/4 ≈ 1/2 and 7/8 - 2/5 ≈ 1/2; students defend each vote.
A recipe uses 3/4 cup milk and 2/3 cup water; estimate the total before measuring and explain why 1 cup is too low.
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