There is an interesting and popular conference of Pr. Albert A. Bartlett on YouTube about the exponential function.

Most of the conference is about population growth and oil and coal reserves, but in the first part, Bartlett exposes some useful properties.

• The doubling time of a quantity growing at a constant rate r is 70/r;
• The growth in any doubling time is greater than the total of all the preceding growth (think about the rice grains on the chess board);
• Over a human lifetime (70 years), a quantity is multiplied by a factor of 2^r.

Towards the end he cites Eric Sevareid’s Law: “The chief source of problems is solutions.” (CBS News 29 December 1970).