Mental Calculation. In Public School of S. A. Rachinsky - Nikolay Bogdanov-Belsky (1895)

Never seen this before. It’s incredible!

I would never have guessed that result! What a weird coincidence.

I'm sure there are lots of little coincidences like this.

Taking x=10, the top of the fraction simplifies to:

5x^2^ + 20x +30

The rest of the solution is left as an exercise for the reader.