Ageologist finds an old rock and wants to determine its age using rubidium-strontium dating. this is possible because 87rb, which has a half-life of 4.75×1010 y, undergoes β − decay and becomes 87sr. the geologist determines that the ratio of 87sr to 87rb is 0.075. assuming there was no 87sr in the rock when it was formed, determine the age of the rock. age of rock = (no response)