Online learning platform Sophia said it has purchased Guaranteach, an online math tutoring business.

Sophia CEO Don Smithmier said in a statement the acquisition will make his company become "the web's richest resource for free, credible math tutorials ranging from basic arithmetic to college algebra" for students and teachers.

"The result will be a powerful platform for differentiated instruction and blended learning," Smithmier said.

Baltimore-based Guaranteach had charged schools for its service on a pay-per-use basis, but after the acquisition, Sophia said the content will be available to individual users for free. Sophia said the technology can be licensed to K-12 districts and higher education institutions.

Guaranteach has about 23,000 videos on math topics and has been used in more than 50 schools. The company said it has software that recommends videos based on what students have watched and changes the difficulty of quiz questions based on a student's response. Sophia said it hopes to use this model to develop tutorials for English and science.

Sophia launched earlier this year. The Minneapolis-based start-up, named after the Greek word for wisdom, has an online platform that allows people to upload educational videos, lectures and slide shows on topics that range from U.S. history to chemistry. Last month, the start-up said it had more than 20,000 users from more than 116 countries.