Consider an element with a m number of 40 and an atomic number of 18... ?

What is the total number of protons and neutrons?? I tried to subtract 18 from 40, which makes 12, and 12 is the number of neutrons, and i thought that the number of neutrons and protons were supposed to be equal (except isotopes) so i multiplied 12 by 2 to get the total but it didnt work. It's in one of my homework's questions, and there are some options, but 24 is not one of them! so i am repeating the question again;Consider an element with a m number of 40 and an atomic number of 18, what is the total number of protons and neutrons?