Final answer:
Rubidium-85 has 48 neutrons.
Step-by-step explanation:
The element with the symbol Rb represents rubidium. Rubidium has an atomic number of 37, which means it has 37 protons. The mass number of rubidium-85 is 85, so it has 85 total nucleons (protons + neutrons). To find the number of neutrons, subtract the number of protons from the mass number. In this case, 85 - 37 = 48 neutrons.