Mark went to an electronic store, purchased 2 cds and 6 dvds, and spent $156. antonio went to the same store, bought 1 cd and 9 dvds, and spent $210. how much does each cd sell for at the store? $12
$15
$18
$22

For the answer to the question above,
Let C be CD's 
Let D be DVD's 

2C + 6D = 156 
C + 9D = 210 

6D = 156 - 2C 
3D = 78 - C 

C + 9D = 210 
C + 3(78 - C) = 210 
C + 234 - 3C = 210 
234 - 210 = 3C - C 
24 = 2C 
12 = C 

The cost of one CD is $12, so the answer is a.

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