An atom of cadmium that has 66 neutrons has a mass number of

113

Explanation:

We are given that the atom of cadmium has 66 neutrons:

  Number of neutrons in Cd  = 66

Now;

 Mass number  = ?

The mass number of an element is the sum of the number of protons and number of neutrons

     Mass number  = number of protons + number of neutrons

 By the position of cadmium, it has an atomic number of 47 and this is the number of protons

     Mass number  = 47 + 66  = 113