Answer:
$0.54
Step-by-step explanation:
Given:
The jars of peanut butter that I buy cost $7.20 each and normally contain 500 grams.
At present, however, each jar contains an extra 20% free.
And additionally, the supermarket has a "buy 3 jars get another one free''.
Question asked:
By how much is the cost per 100 grams of peanut butter less than it normally is if I take full advantage of the supermarket's offer today?
Solution:
Cost of a jars of peanut = $7.20
Normally, weight of peanut butter in one packet = 500 gram
At supermarket offer, each jar contains an extra 20% free, means a packet of peanut butter contains = 500 + 20% of 500 
And extra benefit = buy 3 jars get another one free, means we will get 4 jars by just giving price of only 3 jars.
That means, total weight of peanut butter you are getting by giving 3 packet's price = Each jar weigh
number of jars
= 600
4 = 2400 gram
<u>Cost of each packet will be same as previously as during the offer;</u> $7.20
Cost of 3 packet's peanut (2400 gram) = 
Now, we will calculate cost per 100 grams of peanut butter :
<u>During offer:</u>
Cost of 2400 grams of peanut butter = $21.6
Cost of 1 gram of peanut butter = 
Cost of 100 gram of peanut butter = 
<u>Normal day:</u>
Cost of 500 grams of peanut butter = $7.20
Cost of 1 gram of peanut butter = 
Cost of 100 gram of peanut butter = 
We found that cost of 100 gram of peanut butter in normal day is $1.44 while cost of 100 gram of peanut butter during offer is $0.9 means cost per 100 gram during offer is ($1.44 - $0.9 = )$0.54 less than cost during normal day.
<u>Conclusion:</u>
Therefore, cost per 100 grams of peanut butter is $0.54 less than it is normally if you take full advantage of the supermarket's offer today.