Question 769353
Let the cost of 1 CD to be x whereas the cost of 1 DVD to be y.
{{{4x+3y=120}}} ---> Equation 1
{{{y=x+12}}} ---> Equation 2 ... Substitute {{{y=x+12}}} into Equation 1
{{{4x+3(x+12)=120}}}
{{{4x+3x+36=120}}}
{{{4x+3x=120-36}}}
{{{7x=84}}}
{{{x=84/7}}}
{{{x=12}}} ... Substitute {{{x=12}}} into Equation 1
{{{4(12)+3y=120}}}
{{{48+3y=120}}}
{{{highlight(3y)=120-48}}} ---> 3y is made as the subject of the equation because the cost of 1 DVD is y, thus the cost of 3 DVDs is 3y.
{{{3y=72}}}
Thus, the cost of 3 DVDs is $72.