count each full block and every block that is more than half full as 1 each, ignore every block that is less than half full.
I got exactly 50 using this method before looking at the available answers.
In multiple choice math problems try to ignore the possible answers until you have determined what you think it ought to be. If the answer you found is in the list you have more confidence you are right, if the answer you got is not in the list you know you did something wrong... just a hint :)
