"Hamamura Nagisa no Keisan Nooto" (Hamamura Nagisa's Calculation Notes), volume 9 by Aoyagi Aito, featuring very profitable application of mathematics.
There are many skills that involve varying levels of math, but probably none as dependent on math as gambling. This volume included a chapter on permutations and combinations, and proceeded to apply these concepts at a casino in the next chapter. It explained why carefully computing probabilities is a much better strategy than relying on luck, and why martingale betting strategy might not be a good idea. It was a gentle and practical introduction to these topics.
All volumes in this series have tried to integrate some bits of story with math topics, and this volume have done it especially well with the various romatic relationships. The one with negative numbers and inequality brought life to an otherwise extremely simple topic, and the one with probabilities elevated Nagisa from being just a math genius to also an expert in psychology (at least for that one particular topic). Storywise, this is possibly the best volume yet.
I am glad this series is still going strong, and continues to give new spins to familiar and unfamiliar math topics.
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Next (2019-06-02): "Imouto sae Ireba Ii" volume 12 by Hirasaka Yomi