Let us assume we do not know any mathematics. You can still figure it out conceptually. We know that 1/8 of our sample is carbon 14, and 7/8 of it is carbon 16. That is, our sample once has 100% carbon 14. It took 8000 years to go from 100% to 50%. It took another 8000 years to go from 50% to 25%. And finally, it took 8000 years to go from 25% to 12.5%. Three 8000 years, would be 24,000 years. Decay of radioactive material follows an exponential function. Whenever you know the half-life of a substance and the percentage of your sample that substance consists, say x%, you can solve the equation x/100=(1/2)^k for k. Then, you multiply the half-life by k to see how old your sample is. I hope this helped.