First we find the number of boxes we need to buy 180 pens.
# of boxes = Total # of pens needed ÷ # of pens per box
# of boxes = 180 ÷ 12
# of boxes = 15
Cost of 180 pens = # of boxes x cost per box.
We know there are 15 boxes and each box costs $5.80
Therefore,
Cost of 180 pens = 15 x $5.80
Cost of 180 pens = $87