math functions

Question

I cannot figure out how to take a number from 1 to a 1000 and have the appropriate ending put on it. The last >=4 to 20 is th and ...the 3 ending is rd and the 2 is ND and the 1 is st I think the mod function has a role in this. But how do you tell it to take the number from input figure out the ending and apply accordingly EXAMPLE WOULD BE 191ST OR 823RD OR 498TH 982ND THIS IS MY PROBLEM??????? ANYONE OUT THERE WHO CAN HELP ME PLEASE DO SO. I HAVE SPENT HOURS UPON HOURS AND CANNOT GET IT. PLEASE HELP.