How many neutrons does an element have if it’s atomic number is 49 and it’s mass number is 150?

The atomic number (Z) is the number of protons an element has. This is what actually gives the chemical identity. (While you don't need to know it, you can find that the element is Indium.)

The mass number (A) is equal to Z + N, where N is the number of neutrons. Given a mass number of 150, you have 150 = 49 + N, or N = 101.

So there are 101 neutrons.

And if you wanted to know, this would be the isotope indium-101, which, according to Wikipedia, has a half-life of about 15 seconds!

(Image Credit: Hyperphysics, Georgia State University)