The word "algebra" has 7 letters but only 6 distinctly different letters. The number of permutations of 7 different objects is 7! (=7*6*5*4*3*2*1)=5040. The two a's means that half this number would contain a reciprocal arrangement, because either 'a' could be positioned where its duplicate is. Therefore, the number of permutations is halved: 2520.