Don't be confused about all the information. You only need to know that the two plates have the same weight and that the bar have some weight too.
If we add two plate to the bar, the weight is 115 (look the table)
Now, if we write this with an algebraic expression, we need to define, some symbol that represents the weight of one plate, and some symbol that represent the weight of the bar, for example,
m = the weight of one plate
M = the weight of the bar
So, when we add two plates, we have 115, that means
2m + M = 115 (*) (a linear equations with 2 variables that we need to know, so it is necessary consider more information)
If we look that with 4 plates the total weight is 185, i.e.
4m + M = 185 (**)
Then, with this it is sufficient to know the values for m and M.
if we substracts (*) to (**) we have
2m = 70, then m = 35.
then, in (*) we replace this value
70 + M = 115, therefore M = 45.
This corresponds to C.
