Question

A DVD costs twice as much as a music CD. Jack buys 2 DVDs and 2 CDs and spends $45. Determine how much one CD costs, in dollars. [Only an alegbraic solution can receive full credit]

Answer 1

Let $d=$ the cost of a DVD
Let $c=$ the cost of a CD

We have
$d=2c$
and
$2d+2c=45$

We can replace $d$ with $2c$ in the second equation and get
$2(2c)+2c=45$
$\rightarrow 4c+2c=45$
$\rightarrow 6c=45$
$\rightarrow c=7.5\rightarrow\ 7.50$