The graph shows the carbon-14 left in a sample against its age. A charcoal sample has one eighth of the carbon-14 of a living piece of wood. Which point of the curve fits this sample?

Fig. 1: A curve of the carbon-14 left in a sample against its age in years, falling by half every 5730 years from 100 per cent at age 0. Three points are marked in red: P at an age of 5730 years, Q at 11460 years and R at 17190 years.