Here's a little sample of what is keeping me so busy right now - third grade math, aligning things to the Common Core standards.
This is a complete lesson (4 pages) about area for grade 3 math. It's not the initial lesson - it assumes the student already knows how to find the area of a rectangle. (Sorry - there is no answer key at this time; it hasn't been made yet!)
The lesson (worksheets) is specifically about these two standards:
7.c. Use tiling to show in a concrete case that the area of a rectangle with whole-number side lengths a and b + c is the sum of a × b and a × c. Use area models to represent the distributive property in mathematical reasoning.
d. Recognize area as additive. Find areas of rectilinear figures by decomposing them into non-overlapping rectangles and adding the areas of the non-overlapping parts, applying this technique to solve real world problems.