If we pay $8.28 for 18 stamps we can find the price of one stamp by dividing by 18.
So, the price of one stamp will be
$8.28 / 18 = $ 0.46
To find the price of 12 stamps we have to multiply by 12 and finally find
$0.46 * 12 = $5.52
Hence, the cost of 12 stamps is $5.52