16. What is the relative age of the layers in Figure 4 ? (oldest youngest) 17. Layer B has a 1:3 parent-to-daughter ratio of U235 (half-life =713 million years). What is the age of this layer? 18. Layer D has a 1:7 parent-to-daughter ratio of of K40 (half-life =1.3 billion years). What is the age of this layer? 19. What is the age range of Layer N? 20. What can be done to better constrain the age range of this layer? 21. These are the fossils found in Layer L. Based on the fossils you see present (see the last page for fossil age ranges), what is the age of this layer? 22. These are the fossils found in Layer T. Based on the fossils you see present (see the last page for fossil age ranges), what is the age of this layer?.