If you want to practice skills like these, try:<p>Street-Fighting Mathematics [1] and 
The Art of Insight in Science and Engineering by Sanjoy Mahajan (MIT) [2].<p>Free downloads via:
[1] <a href="https:&#x2F;&#x2F;mitpress.mit.edu&#x2F;books&#x2F;street-fighting-mathematics" rel="nofollow">https:&#x2F;&#x2F;mitpress.mit.edu&#x2F;books&#x2F;street-fighting-mathematics</a>
[2] <a href="https:&#x2F;&#x2F;mitpress.mit.edu&#x2F;books&#x2F;art-insight-science-and-engineering" rel="nofollow">https:&#x2F;&#x2F;mitpress.mit.edu&#x2F;books&#x2F;art-insight-science-and-engin...</a>