A neutral atom of potassium (K) has an average mass of 39 amu and 19 electrons. How many neutrons does it have?

10

19

20

58

I believe the answer is 19

Answer is 20. Since potassium has 19 electrons you know that it then has 19 protons because there is no charge change. So to get the number of neutrons, you take the average mass of 39 and subtract the number of protons and that leaves you with 20 neutrons.