Afossilized leaf contains 8% of its normal amount of carbon 14. how old is the fossil (to the nearest year)? use 5600 years as the half-life of carbon 14.

Elapsed time = half-life * log(beginning amount / ending amount) / log (2)
elapsed time = 5,600 * log (100 / 8) / log (2)
elapsed time = 5,600 * log (12.5) / log (2)
elapsed time = 5,600 * (1.096910013 / 0.30102999566)
elapsed time = 5,600 * 3.6438561898
elapsed time = 20,405.5946628585
elapsed time = 20,406 (to nearest year)
 
Source:
http://www.1728.org/halflife.htm