Each CD is sold for $12.
Let $x be the cost of one CD and $y of DVD.
Thus, for Mark, 2x+6y= $156 --------- (1)
and for Antonio, x+9y= $210 ---------- (2)
From equation (1) y= (156-2x)/6, , put it in equation (2)
x+9 [(156-2x)/6]= 210
⇒ x+ 234-3x = 210
⇒2x= 234 -210
⇒x= 24/2
Thus x= $12, thus cost of CDs is $12 ie CD were sold for $12.
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