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
b = 2 a – 35
c = a – 100
a + b + c = 525
b > a > c
b. We are given three equations with three unknowns. So we can solve the problem.
a + (2 a – 35) + (a – 100) = 525
a + 2 a – 35 + a – 100 = 525
4 a – 135 = 525
4 a = 660
a = $165