An element has 5 protons, 6 neutrons, and 5 electrons. What is the mass number?
Mass number = protons + neutrons
The mass number of an element is the sum of the protons and neutrons. The number of neutrons for a given element is the only number that can change and still have the identity of the element stay the same, (because the atomic number is the number of protons).
In this case the mass number would be 11.