ANSWER FRAME

The NNT is just the inverse of the difference in risk of an event:

NNT = 

1/(RateA-RateB)

Here, NNT of ClotItNot  =

 1 / ( .9 - .8 ) = 

1/ (0.1) = 10. 

That is, 10 patients need to be treated to realize one life saved at a year post-MI.  Click for more info on NNT.