Students develop fluency with addition and subtraction facts within 20 and use place-value reasoning to add and subtract one-, two-, and three-digit whole numbers within 1,000, with and without regrouping, including partial sums, partial differences, and mental work with multiples of 10 and 100. They estimate sums and differences by rounding to the nearest ten or hundred, find missing numbers and addends using inverse relationships, and represent computation with equations, models, number lines, expanded form, and standard algorithms. They apply these skills to one- and two-step addition and subtraction problems involving totals, differences, comparisons, remaining amounts, and unknown parts, while checking that solutions are reasonable.