Lesson objective: Extend understanding of estimation to determine reasonability of sums and differences in real-world contexts.

Students have prior knowledge of estimation from 3.NBT.A.1. This prior knowledge is extended to addition and subtraction as students determine the reasonability of an estimated sum or difference. A conceptual challenge students may encounter is that rounding values will not always result in a reasonable estimate.

The concept is developed through work with a number line, which supports understanding of estimation as using close, benchmark numbers.

This work helps students deepen their understanding of number because students compare estimated sums to actual sums and determine which solution makes sense in the given scenario.

Students engage in Mathematical Practice 8 (Look for and express regularity in repeated reasoning) as they use their knowledge of rounding and apply it to a new situation. They determine estimates and reasonability of calculations with sums and differences.

**Key vocabulary:**

- estimate
- front-end estimation
- overestimate
- reasonable