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