Tammy picks 10 trees x 12 oranges/tree = <<10*12=120>>120 oranges per day.
Tammy sells 120 oranges/day / 6 oranges/pack = <<120/6=20>>20 6-packs of oranges a day
Tammy sells 20 packs/day x 7 days/week = <<20*7=140>>140 6-packs a week
In 3 weeks Tammy sells 140 packs/week x 3 weeks = <<140*3=420>>420 6 -packs
Tammy makes 420 packs x $2/pack = $<<420*2=840>>840
#### 840