I believe very few men who have not trained pull ups would be able to complete even one pull up. I've heard this is the case.
The Royal Marines have a basic fitness standard which you must attain before you can even qualify to join and start training (there is a three day selection process)- whilst the push up and sit up numbers are quite high, I believe the pull up numbers are relatively low- only a minimum of six or so I think (though these are strict pull ups, no cheating, and done as part of a battery of fitness tests). I think this helps illustrate just how fit and strong you'd need to be (the Royal marines are considered one of our elite units) to complete 100 pull ups- even in groups of ten or so. With none of that silly 'kipping' nonsense, obviously.
I've recently added pull ups (or more accurately in my case,
attempted pull ups), but with my 210lb or so, it's a steep learning curve. I've already taught myself the art of strict push ups, and have gone from being able to do none, to doing 15 or so at a time, usually I do five or six sets of ten.
As with all bodyweight exercises, the heavier you are, the harder it is initially, but it gets easier!
I suspect this challenge is a way off from my current levels- the pull ups are still a no-no, the rest I could do, but with the rest periods, it could take a while!
I also prefer full sit ups to crunches, though I'll do the latter from time to time.