Portaankorva-Koivisto P, Viholainen A. Collaboration, problem solving and other 21st century skills for teaching of mathematics at secondary level. lumatb [Internet]. 2022 Jun. 7 [cited 2025 Jul. 6];7(2):86–102. Available from: https://journals-21.hulib.helsinki.fi/lumatb/article/view/1806