a. let us say that:
a = cost of phones with camera
b = cost of phones with camera and mp3
c = cost of phones without any features
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b = 2 a – 35
c = a – 100
a + b + c = 525
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b > a > c
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b. We are given three equations with three unknowns. So we can solve the problem.
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a + (2 a – 35) + (a – 100) = 525
a + 2 a – 35 + a – 100 = 525
4 a – 135 = 525
4 a = 660
a = $165