OR.2.1. Number and Operations: Develop an understanding of the base-ten numeration system and place-value concepts.
2.1.2. Understand and apply base-ten numeration, and count in multiples of one, two, five, ten, and one hundred.
OR.2.2. Number and Operations and Algebra: Develop fluency with addition facts and related subtraction facts, and with multi-digit addition and subtraction.
2.2.1. Apply, with fluency, sums to 20 and related subtraction facts.
2.2.4. Select and apply efficient methods to estimate sums and differences or calculate them mentally depending on the numbers and context involved.