An analysis of the rock shows that the relative number of Sr87 and Rb87 (t½ = 4.7 × 1010 year) atoms is 0.05. What is the age of the rock? Assume all the Sr87 have been formed from Rb87 only.

Atoms of Sr Atoms of Rb=0.05 or Atoms of (Sr + Rb)atoms of Rb=1.05 so, Initial no. of atoms of RbPresent no. of atoms of Rb=1.05 t = 2.303Klog(n0/n) =2.3030.693×4.7×1010 log (1.05) = 3.28 × 109 years