Every tray needs 80 cookies and she's making 3 trays so 8*3 = <<80*3=240>>240
Every box of cookies has 60 cookies. She needs 240 cookies so 240/60 = <<240/60=4>>4 boxes
The boxes cost $3.50 each and she needs 4 boxes so 3.50*4 = $<<3.50*4=14.00>>14.00
#### 14