I've come up against another difficulty in my endeavor for learning basic chemistry.

As an example, I'll give this equation:

Element___Isotope___Mass Of Isotope_____Proportion___Mass X Proportion___Sum

___________52/24________52.000_____X_____A_____=___52.000 X A

Cr________________________________________________ __________+= 51.996

___________51/24________51.000_____X_____B_____=___51.000 X B

If you could just envision the problem in the absence of the lines. I tried to make the problem as least confusing as possible and I apologize for the amateur construction of my diagram. I've done a number of problems similar to this and I still can't seem to grasp the concept of what's really going on here. I'd really appreciate some feedback on this as to this.

Thanks.