173 million is just an odd of winnig the jackpot, it's not actually the total number of combination. Like 20$  scratch off has a odd of 3.2 but every 3rd ticket is not a winner, so is with the mega million jackpot. If you wanna really do the math then, for every 1st number there is possibility of 56 number, second number 55 (coz the first number won't be repeated). third numbger 54. 4th number 53, 5th number 52 possible number. And for mega ball you have separete 46 times number of combination. so the total number of combination would be= (56*55*54*53*52)*46=21,085,384,320.. So you will have 21 billions of combination. By the way somebody in marryland got the winning ticket so, should not worry about it anymore :p