You purchased 150 feet of chain that normally sells for 0.15 per inch you were charged 22.50 for the sale was the price correct if not what is the correct price an what mistake was made

The appropriate price is
  (150 ft)*(($0.15/in)*(12 in/ft)) = $270.00

Apparently, the factor (12 in/ft) was left out of the computation, so the charge was for $0.15 per foot.

a) The price was not correct. The correct price was $270.00

b) The mistake was charging by the foot rather than by the inch.

Logical equivalent of the give question is its contrapositive