The first cart contributes 3 feet in length to the nested carts and the remaining 11 carts add a constant length to the nested carts. Let that length be x.

So,

3 + 11x = 16.75

11x = 16.75 - 3

11x = 13.75

x = 13.75 / 11

x = 1.25 feet

So, the length of each cart that is inside another = 3 - 1.25 = 1.75 feet

oh my gosh! thank you so much, you don't know how much that helped but i have a few more questions please. if the length of each cart that is inside another cart is 1.75 feet, what would be the length of a 20 cart stack.