the reson of choosing 10 is obvious, but why e=2.7...?
the answer is that it back to 300 years or more ago. to Euler (which e comes from his name). the function e^x is the only function that its derivative (and consequently its integral) is itself. {e^x'=e^x). no other function has this charasteristic. the number e could be achieved by several numerical and analytical methods, more often infinite summations.
this number has more important rule in complex analysis.
marti