Answer:
Step-by-step explanation:
There's a small trick here. You have to double the thickness. Why? Because the outside diameter goes through the thickness twice.
Inside diameter = outside diameter - 2 * thickness
outside diameter = 1 1/4 inches
thickness = 5/16
Inside diameter = 1 1/4 - 2 * 5/16
inside diameter = 5/4 - 10/16
Inside diameter = 5/4 - 5/8
Inside diameter = 10/8 - 5/8
Inside diameter = 5/8
