You're in luck i'm feeling the mood to do your homework :)
2) The table would change, the number of hydrogen and oxygen atoms would be the same ALWAYS.
3) 15 Hydrogen Atoms and 5 Nitrogen Atoms (5*3=15, 5*1=5)
4) Not so great with System of Equations, I feel like there is a value missing. It'd be much easier if I was given how many people went.