Monday, March 5, 2012

Problem Solving: Stoplight

The Stoplights problem challenges students to figure out how many different stoplights they can create with 3 different color linking cubes if each stoplight must have one cube of each color. 

Use linking cubes so that students can assemble all possible stoplights. You may choose to have students use any three different colors so that each student pair has enough linking cubes to assemble all possible combinations using one of each color to create the stoplight.

In these kinds of organized counting problems, it is important to question students about how they know they have found all of the possible combinations.  A first grader once responded that he knew he had them all because each color gets to be on top two times.  What a great insight!

Download the Stoplights problem student handout.


  1. Nice! I like this simply-defined, interesting problem with multiple possible solution paths, and levels of solution.

    Perhaps you are giving away the solution by having the right number of boxes on the worksheet? You could put 8, or 10 solution spaces, or some other "plausible" number, and challenge students who say they can only find 6.

    1. Great suggestion! Actually, my favorite method is to use a blank piece of paper so that students organize and present their solution with no prompting.