What do you mean it works both ways?
Subtraction is where the magic really began to happen. Watching students use number lines to subtract with regrouping was like watching Olympic gymnasts defy gravity as they flip and twirl on the balance beam. You found yourself watching students and thinking, “Go for it! “Then applauding and saying, “She nailed it!
He stuck the landing!”
Workbooks provide lessons on using fact families and lessons on the relationship between addition and subtraction, but I rarely if ever see students apply those relationships outside of the book. If you want kids to understand properties and use them, have frequent number line talks with the whole class.
Draw a blank number line on the board and ask the students, “If you needed to subtract 152 – 99 on a number line, where would you place the numbers and why?”
Some students will place 152 on the right side of the number line and count down 99.
152 – 99 = ___
Draw another blank number line.
Some students will place 152 on the right side of the number line and count down until they get to 99.
152 – ___ = 99
Draw another blank number line.
Some students who recognize the relationship between addition and subtraction will place 99 on the left side of the number line and count up to 152. Some students who want to “experiment” will unknowingly try the same strategy and “discover” the relationship between addition and subtraction and find the difference between 99 and 152.
99 + ___ = 152
