Answer :
First we have to give names to the quantities that each lady has.
Number of CDs that Erica has = E
Number of CDs that Kim has = K
The algebraic expression (actually an equation):
E = K - 4
Number of CDs that Erica has = E
Number of CDs that Kim has = K
The algebraic expression (actually an equation):
E = K - 4
[tex]E\ \rightarrow\ the\ number\ CDs\ of\ Erica\\\\K\ \rightarrow\ the\ number\ CDs\ of\ Kim\\\\E=K-4[/tex]