The perimeter of an object is the sum of all sides.
a book is a rectangle which has 4 sides. 2 sides would be the length and 2 sides would be the width.
Set up the equation:
70 = 20 + 20 + x + x
Simplify
70 = 40 + 2x
Subtract 40 from both sides
30 = 2x
Divide both sides by 2
X = 15
The width of the book is 15 cm