If you haver 5 grams of carbon-14, whose half-life is 5730 years, How much carbon-14 will be left after 4,000 years and 8,000 years?

Any help would be highly appreciated.

A simple search gives this:
http://www.ridgecrest.ca.us/~do_while/sage/v4i10f.htm
http://www.1728.com/halflife.htm?b9=15