Have you seen a question like this before?

How can you solve this problem without drawing a picture on paper?

Use one of the resources below to solve the problem.

• Use this interactive tool to construct triangular numbers until you can see the pattern to answer the counter question from above.

• ADVANCED: This is a teaching page that REALLY gets into the math behind these types of problems. Use the formula and see if you can get the right answer.

• Check out this triangular number chart.  Click on getting started. When you have a solution and a written statement about what you did to solve the problem, check the “solution” button to check your work.

• Connection: Research the original Light Brite, a 1960s toy. Come up with some unique idea on how teachers can use these toys to teach math, science, and art.

Image source: https://nzmaths.co.nz/sites/default/files/TriangularNumbers.pdf