Sum of hats Puzzle

The problem is taken from here :
Description goes like this :
There are 3 people Abel,Bill and Clark.Three of them have numbers written on their hats.One can only see the numbers written on others hats and can not see the number written on his own hat. Sum of numbers on any two 2 hats is equal to the number on the third hat.Now the following event occurs
1. Abel was asked about the number on his hat.He replied "Don't Know"
2. Bill was asked about the number on his hat.He also replied "Don't Know"
3. Clark was asked about the number on his hat.He also replied "Don't Know"a
4. Abel was asked again ,to which he repplied "50"

Now the question is how did he know it.And what are the numbers on other people's hats.