Answer: 96.2 billion years.
Step-by-step explanation: Half life of Rubidium-87 = 48.1 billion years
Half life is the time required to decay half of the radioactive material. Thus if 100 g of sample is present, 50 g of sample remains after half life and 50 g decays.
For 25 percent parent isotope and 75 percent daughter to remain, means 25 g of parent isotope is present and 75 g of daughter isotope is present.
This will happen after two half lives i.e 96.2 billion years and after second half life, out of 50 g , 25 g will decay and 25 g will remain.
Thus total 75 g is decayed as daughter isotope and 25 g remains as parent isotope.