Grade 2 · Number
Benchmarks of 25, 50, and 100
Benchmarks are the landmarks of the number line. A student who can quickly locate a number relative to 25, 50, and 100 has a rich quantitative sense that makes estimation fast and accurate. Is 73 more or less than half of 100? Much more. Is 38 close to 25 or 50? Closer to 25? No: 38 is 13 from 25 and 12 from 50, so actually closer to 50. These landmark-based comparisons build the mental number line that underlies all of mental arithmetic.
LESSON VIDEO
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Lesson video
A short walkthrough to play in class or assign for flipped/at-home viewing.
WHAT STUDENTS WILL LEARN
✓Identify and use 25, 50, and 100 as benchmark numbers for estimation and comparison
✓Determine whether a given quantity is closer to 25, 50, 75, or 100
✓Use personal referents (e.g., a quarter-dollar, half a loaf) to ground the benchmarks
✓Estimate quantities using benchmark reasoning
✓Connect benchmarks to real-world and cultural contexts: seating at feasts, sharing food
BUILDS ON