[dekhn]

I learned the multiplication using addition and a lookup table in a class taught by Huffman (of Huffman compression fame). You weren't allowed to use a calculator on the test.

But my absolute favorite trick is base conversions, https://www.khanacademy.org/math/algebra2/x2ec2f6f830c9fb89:... with some practice you can do approximate base conversions (power to 2 to power of 10 or e) in your head