How tall is a tower of 100 pennies

A stack of 100 pennies, one on top of the other, is 15.5 centimeters tall. 

a penny is 1.55 mm thick, so a stack of 
100 pennies would be 155 mm thick or 15.5 centimeters. 

This means that a stack of 100 pennies is 6.1 inches in height.