Charge to mass ratio

[vivatix]

Can someone explain how they are determining the charge to mass ratio here?

For choice B, Sodium: charge to mass is 1/11 and calcium is 2/20. The two are pretty close but they are not the same.

When I did this question, I compared the mass of the 2 atoms and the charge of the two atoms. So, I thought that C would be the correct answer since the two atoms have the same mass. What is wrong with my method? How to tackle such problems? Any tips, advise?

Thanks

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[bee17]

You have your mass numbers and atomic numbers mixed up. The top number is the mass number. Therefore, the charge to mass ratios are 1/12 and 2/24, which are the same.