Let's just take a moment to THINK ABOUT WHAT WE KNOW.
If Tessie wanted (to pay for) 3 cans of veggies, the store would give her one free--so four cans, ΒΎ of which she pays for.
If you multiply this basic fact by 8, you see that would be 32 cans total, and ΒΌ of them would be free.
Of the 32 she takes home (very heavy), she pays for 24, and gets 8 free.