What we should all stand in awe of is that at Fermi's insistence, the team decided that before the test shot, they should try to work out whether the "chain reaction" they hoped to initiate would ever stop - i.e. would the first bomb be the last, because it destroyed the entire planet. The team members set to the task (headed by Teller) did all this without computers. See "Edward Teller - The Real Dr. Strangelove" by Peter Goodchild (pub 2004) at page 104. The side bet is recorded (with Teller and Oppenheimer's predictions as well as Rabi's) lower down on the same page.