If you want to do this and make it pretty, you'll come up with some nice ways of re-using different cases for once you get into double-digits.
If you want to strongarm it, all you need is a switch and 50 case statements.
Just load the input into an integer, then do a switch. Here's an example: http://www.cplusplus.com/doc/tutorial/control.html
It's at the bottom of the page. Do NOT forget to put in the break; for each case otherwise it will fall through and print everything after that case.
Good luck, if you need help post again with your code.
take a look at where you can re-use switch statements you've already created.
For instance
twenty-one is a combination of printing twenty, and then one. Both of which you already have from other cases. So instead of a case for 21,22,23,24,25. You can combine your case for 20, and then for one, two, three, four, five, ect.