LearnZillion lesson plan. (Jeffrey MacMillan/Jeffrey MacMillan)

One school day a few years ago, Eric Westendorf, then-principal at E.L. Haynes Public Charter School in Northwest Washington, realized he didn’t fully understand a basic middle-school math concept: What it really meant to divide whole numbers by fractions.

Westendorf was sitting in on a sixth-grade math class, where a teacher was explaining the idea to her students. Dividing 4 by 2 is like saying “how many sets of 2 are there in 4?” she explained. Dividing 4 by 1/2, is like saying “how many 1/2’s are there in 4?”

It was an elegant explanation, Westendorf said — one he wanted to share with other teachers who might be struggling to explain the same notion to their students. So he decided to build a Web site stocked with videos of teachers explaining basic math and English concepts — especially the ones students are required to learn under statewide curriculum standards — that other teachers can use supplement their lesson plans.

“There’s a particular pain point in elementary school math,” Westendorf said. “An elementary school teacher probably didn’t go into teaching because he or she loved math specifically — they probably love kids and they’ve had amazing experiences with kids ... [teaching math can be] very daunting for teachers who may have anxiety about their own understanding.”

LearnZillion has free and premium options; teachers can access free video lessons, and practice problems, and can also use the system to track student assignments. A math video might describe “ratios,” and an English one might explain “imagery” in literature. For a fee — of roughly $5,000 per school, usually paid by the district — teachers can create their own content and compare student progress across classrooms, among other features.

Founded in 2011, LearnZillion began when Westendorf asked three teachers at the charter school to record their own lessons. Initially, he cobbled together a site using a combination of YouTube, Google docs and Web hosting service Weebly.

Today, LearnZillion is a 28-person tech company based near Dupont Circle, with more than 200 teachers contributing video lessons. LearnZillion employees include several engineers, who develop the site technology, an academic team of former teachers to make the site more user-friendly for teachers, and a sales and marketing team.

About 375,000 users are registered on the site; customers include the District of Columbia Public Schools.

LearnZillion has raised about $9.4 million in venture capital since its founding. Its most recent round, which the company announced earlier this month, included investments from O’Reilly Alpha Tech Ventures, NewSchools Venture Fund, Calvert Social Investment Fund and D.C. Community Ventures.

In his experience as a teacher and a principal, Westendorf said he noticed most teachers using a trial-and-error process to establish the most effective teaching strategies; LearnZillion could help them discover those strategies quicker.

“It’s that same scientific method of what’s working, what’s not,” Westendorf said. Even the teacher who impressed him when describing fractions “definitely didn’t teach it using that model when she was starting.”