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.