We are given
The books thickness is 31 millimeters
we know that
1 millimeters=0.1 centimeters
so, we can write
31 millimeters=31*1 millimeters
so, we can 1 millimeters
we get
31 millimeters=31*0.1 centimeters
now, we can multiply
we get
31 millimeters=3.1 centimeters..........Answer
