An interquartile range is finding the median of the first half of the data set, and the second half, then subtracting the first median from the second one.
First we want to find how much it costs for 1 stamp, so we do 8.28 divided by 18, which equals 0.46. Then, we multiply 0.46 and 12, which is 5.52. Therefore, it would cost $5.52 to buy 12 stamps.
