Yep, I agree with Donna, there is no error except in the title. "Its" is correct if the word is as a posessive, i.e. "He was under its control" or to use both versions in the same sentence, "It's only one of its parts."
It is meant to be misledaing. The majority of people would assume the error was in the numbers and never look at the title.
“Twenty years from now you will be more disappointed by the things that you didn’t do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover.“
Magicians call this "misdirection" and it is a very important part of every magic trick. Causing the audience (or readers) to look away from what is actually important allows the trick to work. Of course those scantily dressed assistants are a big part of it.
Maybe I should have put in a girl in a bikini instead of the numbers ?
Actually, in addition to the its/it's error, there are two other errors also.
The sentence itself is a grammatically incorrect comma splice. Remember your English teacher telling you about that?
And finally, if 80% of the students failed to get it, that means 20% did, so it is merely hard, but not IMPOSSIBLE!