It an element has 13 protons, 13 electrons and 15 neutrons what is its mass number?

28

Explanation:

The mass of an element, also called its atomic mass, can be calculated by adding the number of protons and neutrons.

P + N = mass

13 + 15 = 28

Therefore, the atomic mass is 28.

The number of electrons is generally not considered because they are so light, each of them weighs almost nothing.