you can make the u with an acute accent (ú) in windows by holding down your alt key and typing 0250. (there might be an easier way, but that's the way I know!) Or you can use the little special character input integrated in duolingo. If all else fails you can use your windows character map (usually somewhere in the accessories menu). On most smartphones if you press and hold the letter it will give you a little popup menu of accented versions of that letter.
Usted is also correct. It would be used when speaking to someone more formally, as you would to someone who you don't know or someone to show respect.
why can ticket be both "boleto" and "boleta"? why and when is the gender change used?
I have two choices, both of which are correct. You were not specific as to familiar (tu) or formal (usted). I have both answers.