Oh, 10 minutes ago, it was your dad. Now it's you?
Fill out a blank tax return. 1040EZ would suffice.
As for pencentile, read the homework problem again. I think it's asking for tax bracket, not percentile. To get a feel for brackets, do the 1040EZ, then look at form 1040ES. You will overstate the income if you use $80,000. You want taxable income.