Answer: The other isotope is

Step-by-step explanation:
Beta decay is defined as the decay process in which a neutron gets converted to a proton and an electron.
In this decay process, beta particle is emitted. The emitted particle carries a charge of -1 units and has a mass of 0 units. The released beta particle is also known as electron.
The general equation for the beta decay process follows:

The chemical equation for the beta decay of an isotope that produces Sr-87 isotope follows:

The isotope that undergoes beta decay to form Strontium-87 isotope is Rubidium-87
Hence, the other isotope is
