Cost of five trays of plant = $105
Cost of one tray of plant = $105
5
So, Cost of one tray of plant = $21
Since, each tray contains 12 plants.
So, Cost of 12 plants = $21
Cost of 1 plant = $21
12
Cost of 15 plants =

So, Cost of 15 plants = $26.25
Therefore, Tracy needs to spend $26.25 for 15 additional plants.