FindFit[{{253,20},{273,10},{293,5},{313,2.5},{333,1.2}},a*Exp[b*x],{a,b},x]

Uh oh! Wolfram|Alpha doesn't run without JavaScript.

Please enable JavaScript. If you don't know how, you can find instructionshere.Once you've done that, refresh this page to start using Wolfram|Alpha.