The answer is "tens" but the clue doesn't make sense! The clue is "5 1 5, 7 1 3, 1 1 9, for example". There are spaces between the numbers in each group. Obviously the outside numbers in each set add to ten, but the inside number is always one. If it was a three-digit number It might refer to the place value, but then it wouldn't be printed with spacing like that. It was in a book of Monday NY Times puzzles and has been bugging me a lot! Anybody?