Trisha's pay before taxes is 15 dollars an hour x 40 hours per week x 52 weeks = <<15*40*52=31200>>31,200 dollars
The amount withheld is 20 percent, or 0.2 x 31,200 = <<0.2*31200=6240>>6,240 dollars
Trisha's gross pay of 31,200 - 6,240 = <<31200-6240=24960>>24,960 dollars
#### 24,960