1. treadmill is a piece of equipment in gym OR in the gym? --- "A treadmill is a piece of equipment in the gym" 2. do you use the word "treadmill" or "running machine" to describe that kind of equipment --- here, we use "treadmill" 3. Because "equipment" means all kinds of tools you can use in a gym, so is it correct to say : 3.1a kind of equipment to replace a piece of equipment? ---Did you mean "replace a piece of equipment with a different kind of equipment" 3.2 all kinds of equipment --- this is correct