Let [tex]d=[/tex] the cost of a DVD
Let [tex]c=[/tex] the cost of a CD
We have
[tex]d=2c[/tex]
and
[tex]2d+2c=45[/tex]
We can replace [tex]d[/tex] with [tex]2c[/tex] in the second equation and get
[tex]2(2c)+2c=45[/tex]
[tex]\rightarrow 4c+2c=45[/tex]
[tex]\rightarrow 6c=45[/tex]
[tex]\rightarrow c=7.5\rightarrow\$7.50[/tex]
