For this case we have the following equation:
 c + b = 55
 Where,
 c: number of CDs
 b: number of books
 For c = 30 we have:
 30 + b = 55
 From here, we clear the value of b.
 We have then:
 b = 55-30
 b = 25
 Note: observe in the graph that when the value of c is 30, the value of b is 25.
 Answer:
 the amount Alison may spend on books in March if she spends 30 on CDs is:
 b = 25
 See attached image.
