Benchmarks of 25, 50, and 100
Warm-up
Estimation jar: show a jar with approximately 50 objects. Is this more or less than 25? More or less than 100? About how many? Give students 10 seconds to estimate, then count by making groups of 10. Was your estimate close? What benchmark did you use?
Explore
Benchmark placement: each pair receives 6 number cards (e.g., 17, 34, 51, 68, 82, 95). They place each on a large number line marked with 0, 25, 50, 75, 100 and explain the placement. How do you know 68 is between 50 and 75? It is bigger than 50 but less than 75.
Consolidate
Practice
Students estimate 5 quantities using benchmark jars and record which benchmark they used. Place 10 numbers on the benchmark number line. Exit ticket: is 67 closer to 50 or 75? How do you know?
Exit ticket
Students estimate 5 quantities using benchmark jars and record which benchmark they used. Place 10 numbers on the benchmark number line. Exit ticket: is 67 closer to 50 or 75? How do you know?
Distance from 25: 38 - 25 = 13. Distance from 50: 50 - 38 = 12. Distance from 75: 75 - 38 = 37. Closest to 50, by a margin of just 1. Students who compute these distances are applying subtraction to comparison: a powerful double use of computation.
25 x 4 = 100 invitations per hour. 83 is less than 100, so yes, about one hour. Using the benchmark of 100 (4 groups of 25) makes this estimation immediate.