Problem: My students have such a hard time with unknowns in all positions of math problems. (Common Core standards 1.OA.A.1, 2.OA.A.1)
Solution: Give them plenty of exposure to part-part-whole! It seems so basic, just a way of showing the two smaller numbers that combine to make a bigger number. As shown on the picture, 3 and 6 are the parts, put them together and 9 is the whole. It can be shown with circles (called number bonds or math mountains when turned to point upward), made with cubes, and represented with equations. Look at how part-part-whole helps with an unknown at the beginning of an equation:
Rachel had some pencils. Her teacher gave her 6 more pencils. Then she had 9 pencils. How many pencils did Rachel have to start?
We know the whole is 9 and one of the parts is 6. Using a part-part-whole representation kids can find the missing part either by counting on or by subtracting.