A horribly oversimplified Excel model for modelling defaults

Please, please first read (or skim) my 4 posts on "What an X% default rate should mean to me" -- which cover Prosper.com loans from the perspective of lenders

Default Rates, Part I
Default Rates, Part II
Default Rates, Part III
Default Rates, Part IV

UPDATE: Same file, but slightly more user friendly: don't hotlink it here...