The previous expansion of EdTech as a substitute for traditional learning around the world, the recent full-scale substitution due to COVID-19, and potential future shifts to blended approaches suggest that it is imperative to understand input substitutability between in-person and online learning. We explore input substitutability in education by employing a novel randomized controlled trial that varies dosage of computer-assisted learning (CAL) as a substitute for traditional learning through homework. Moving from zero to a low level of CAL, we find positive substitutability of CAL for traditional learning. Moving from a lower to a higher level of CAL, substitutability changes and is either neutral or even negative. The estimates suggest that a blended approach of CAL and traditional learning is optimal. The findings have direct implications for the rapidly expanding use of educational technology worldwide prior to, during, and after the pandemic.