To keep rabbits away from lettuce garden, she need to set up chicken wire along the garden's circumference. If circumference comes out to be > 20 feet, then yes she will have enough chicken wire to surround the garden. If not , it will be food fest for rabbits. LoL!
Anyways,
Circumference of lettuce garden = pi x diameter of garden
= 3.14 x 6 = 18.84 feet.
Whereas, she has 20 feet of chicken wire.
So she will be left with 1-2 feet of wire.
Better luck for the next time Mr. Bunny :D