Final answer:
The unknown element with a mass number of 54 and containing 24 protons has 30 neutrons.
Step-by-step explanation:
To determine the number of neutrons in an element, you can subtract the number of protons from the mass number. The mass number is the total count of protons and neutrons in an atom's nucleus. Given that an unknown element contains 24 protons, a mass number of 54, and we know it has 21 electrons which does not affect the calculation for neutrons, we can calculate the number of neutrons as follows:
number of neutrons = mass number - number of protons = 54 - 24 = 30.
Therefore, the unknown element contains 30 neutrons.