Question

There are two naturally occurring isotopes of indium: indium-113 and indium-115. How many neutrons are in a single atom of indium-115?
49
66
115
164

Answer 1

There are 66 neutrons in a single atom of indium-115. The atomic number of indium-115 is 49, meaning there are 49 protons. Then the atomic mass is 115, so 115-49 = 66.