Table:
| Years Ended | July 27, 2019 | July 28, 2018 | July 29, 2017 |
| --- | --- | --- | --- |
| Amortization of purchased intangible assets: |  |  |  |
| Cost of sales . | $624 | $640 | $556 |
| Operating expenses |  |  |  |
| Amortization of purchased intangible assets | 150 | 221 | 259 |
| Restructuring and other charges | — | — | 38 |
| Total  . | $774 | $861 | $853 |
Paragraphs:
5. Goodwill and Purchased Intangible Assets
(b) Purchased Intangible Assets
The following table presents the amortization of purchased intangible assets (in millions):
Question:
What was the change in cost of sales between 2017 and 2018?
Answer: 640-556 = 84