It's harder than that. Our food supply has changed; sixty years ago when cows ate grasses, beef naturally had some omega-3s in it. Now that their diet is largely corn, beef brings virtually all omega-6s.
Ditto for "shelf-stable" foods. Omega-3 fatty acids go rancid at room temperature, so many packaged products have had the omega-3-based fats removed so they'll last longer on the shelves.
Source: my wife, who has a PhD in nutrition.