The half-life of a radioactive isotope is the average length of time in which 50% of a sample of the substance undergoes radioactive decay. The original substance is callled the parent isotope and the decay product is the daughter isotope.
Since 25% of the Rubidium-87 remains, approximately two half-lives have passed. After one half-life about 50% remains, and after a second half-life 50% of that 50% which is 25% remains.
Two half-lives = 2 x 48.1 billion years = 96.2 billion years
Rubidium-87 undergoes beta decay to form strontium-87. Beta decay increases the atomic number by one because a neutron turns into a proton and a beta particle, which is a high energy electron that's emitted by the atom.
Rubidium is present in some minerals and can be used to date rocks by the comparison of the ratio of rubidium-87 to strontium-87.