First, you have to know which amount is your 100%.
Let $160 be your 100% (since the question asked for the amount the price decreased from $160), or let $160 be 100units,
$160 = 100units
The amount price decreased:
$160 - $72 = $88
$160 = 100units
$1 = 100/160units
$88 = 100/160units × 88 <Type in calculator>
= 55units or 55% decrease in price
The units sign is to help you understand it better. If you are able to understand without the unit sign, simply replace all "units" with "%".
Hope this helps! :)