Answer:
She spent $38.7 on the remaining boxes.
Explanation:
total number of boxes of pencil bought by Ms. Smith, = 30 boxes
cost of each box = $2.15
fraction of boxes of pencil used = 6 / 15
number of boxes of pencil used =

number of boxes of pencil remaining = 30 boxes - 12 boxes = 18 boxes
Cost of the 18 boxes remaining = 18 x $2.15
= $38.7
Therefore, she spent $38.7 on the remaining boxes.