Answer:
it is given that the cost of the book was $20 less than twice what she spent on a backpack, this implies that:
Cost of book = (2*cost of backpack)-20--------------> [1]
Since, she spent $50 on the book, thus cost of the book =$50
If we substitute this in [1]
50 =( 2*cost of backpack)-20
Rearranging the terms,
50 + 20 = 2*cost of backpack
=> 70 = 2*cost of backpack
=> 70/2 = cost of backpack
=> 35 = cost of backpack
