Select the set of equations that represents the following situation: The cost of eleven CDs and twelve DVDs is $150.62; and the cost of one DVDs and twelve CDs is $116.07. How much do a CD and a DVD cost?

Question

Select the set of equations that represents the following situation: The cost of eleven CDs and twelve DVDs is $150.62; and the cost of one DVDs and twelve CDs is $116.07. How much do a CD and a DVD cost? 
(Points : 3) 
       11x + 12y = 150.62; 12x + 1y = 116.07
       11x • 12y = 150.62; 12x • 1y = 116.07
       11x + 12y = 150.62; 1x + 12y = 116.07
       11x • 12y = 150.62; 1x • 12y = 116.07

Henry · Answer

Eq1: 11X + 12Y = 150.62
Eq2: 12X + Y = 116.07
MultiplyEq2 by -12:
11X + 12Y = 150.62
-144X -12Y = -1392.84
Sum: -133X = -1242.22
X = -1242.22 / -133 = $9.34 = Cost of
1 CD.

Substitute 9.34 for X in Eq2:
12*9.34 + Y = 116.07,
112.08 + Y = 116.07,
Y = 116.07 - 112.08 = $3.99 = Cost of 1 DVD.