In the second part of this two-part episode about lessons learned from my time working in the education technology sector, I wanted to share a very significant quantitative finding to improve learning: what I call the "90% rule". Desirable difficulties is a concept that many know about and try to apply to teaching situations, but there is a question of how difficult one should make things. Surely there is a level at which things are too difficult? In which case, what is the perfect level of difficulty that we should aim for? The secret is this 90% number. Enjoy the episode. *** REFERENCES Eglington, L.G., Pavlik Jr, P.I. Optimizing practice scheduling requires quantitative tracking of individual item performance. npj Sci. Learn. 5, 15 (2020). https://doi.org/10.1038/s41539-020-00074-4 See also the SuperMemo Guru wiki: https://supermemo.guru/wiki/Optimum_interval. SUPPORT If you would like to support Education Bookcast and join the community forum, you can do so at https://www.buymeacoffee.com/edubookcast.