There are two naturally occurring isotopes of indium: indium-113 and indium-115. How many neutrons are in a si?

The atomic number of Indium is 49; this is the number of protons in the nucleus. 'Indium-115' refers to an isotope having atomic m of 115. Since the nucleus of any atom is composed of protons and neutrons only, and these are of equal m, it follows that the number of neutrons in indium-115 is 115 - 49 = 66 (answer B)