The way to get this answer is relatively simple. The first thing you do is multiply the number of each isotope against the number of neutrons in each isotope, which is the number given with each isotope. So, 20 times 19 is 380. 9 times 20 is 180. 52 times 21 is 1092. 19 times 22 is 418. Then, you add them all up. 380 plus 180 plus 1092 plus 418 is 2070.
Finally, you divide that number by 100. So, 2070 divided by 100 comes out to 20.7. The relative atomic mass of element N with the isotopes N-19, N-20, N-21, and N-22 is 20.7.
Atomic weight can be a tricky thing to pin down, but this is an easy way to see how it can be done.
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