Afossilized leaf contains 15% 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.

This is a half life problem.
you'll find this page quite but i'll solve your question anyway.
time = half-life * log [bgng amt / ending amt] / log 2
time = 5,600 * log [100 / 15] / log 2
time = 5,600 * 0.82390874095 / 0.30102999566
time = 5,600 * 2.7369655942
time = 15,327 years

Here, feel free to ask if there are any questions