Chapter 1, Equations and Inequalities - 1.3 Solve Linear Equations - 1.3 Exercises - Problem Solving - Page 23: 72

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Let $x$ = the number of CDs $10-x$ = the number of DVDs The total cost then will be: $15x+22(10-x)=199$ Solve for $x$: $15x+220-22x=199$ $-7x=-21$ $x=3 \rightarrow$ The number of CDs The number of DVDs: $10 - 3 =7$ When the total cost changes to $185$, the equation will be: $15x+22(10-x)=185$ Solve for $x$: $-7x=-35$ $x=5$