How old is a woolly mammoth if 30 percent of its carbon-14 is still remaining?

Carbon-14 has a half-life of approximately 5,730 years, meaning that after each half-life, half of the original amount remains. If 30 percent of the carbon-14 is still remaining, it means that the mammoth has undergone approximately 1.7 half-lives (since 100% to 50% is one half-life, and 50% to 25% is another). Calculating this, the woolly mammoth would be around 11,490 years old (1.7 x 5,730 years).