8. The figure below shows the composite develop- mental skill level of chessmasters at different ages as determined by their performance against other chessmasters. (From "Rating Systems for Human Abilities," by W.H. Batchelder and R.S. Simpson, 1988. UMAP Module 698.) (a) At what age is the "typical" chessmaster play- ing the best chess? (b) At approximately what age is the chessmas- ter's skill level increasing most rapidly