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added best data science learning tutorial

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free-courses-en.md

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 * [Advanced Data Mining with Weka MOOC](https://www.cs.waikato.ac.nz/ml/weka/mooc/advanceddataminingwithweka/)
 * [Data Cleaning by Rachael Tatman at Kaggle](https://www.kaggle.com/learn/data-cleaning)
 * [Data Mining with Weka MOOC](https://www.cs.waikato.ac.nz/ml/weka/mooc/dataminingwithweka/)
+* [Data Science interview questions](https://www.youtube.com/playlist?list=PLZoTAELRMXVPkl7oRvzyNnyj1HS4wt2K-) - Krish Naik
 * [Data Visualization by Alexis Cook at Kaggle](https://www.kaggle.com/learn/data-visualization)
 * [Introduction to Python for Data Science](https://www.edx.org/course/introduction-python-data-science-microsoft-dat208x-3)
 * [More Data Mining with Weka MOOC](https://www.cs.waikato.ac.nz/ml/weka/mooc/moredataminingwithweka/)