In the second week, 15 x 3 = <<15*3=45>>45 bags of chips were sold.
So there was a total of 45 + 15 = <<45+15=60>>60 bags of chips sold on the first and second week.
Thus, a total of 100 - 60 = <<100-60=40>>40 bags of chips were sold on the third and fourth week.
Therefore, 40/2 = <<40/2=20>>20 bags of chips were each sold on the third and fourth week.
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