I learned about this great tool from Kim Sutton from Creative Mathematics! I love how this activity is so versatile and has been a great addition to my RTI groups! Students love trying to figure out what the code is and how they can solve the computations! In addition, you can use this tool with any operation and it is challenging!
To create these boards I simply used Washi Tape and a small sized poster board folded in to four or nine equal parts. I laminated each poster board and use sticky notes to create the numbers for the board. Then I write the problems on the board and students are ready to solve!
Good Luck and Keep them Challenged!