I think the answer is: each shelf is 76 inches, if you take 76x5 it comes out to 380 so that would leave the 20 inches left over, hope that makes sense
THE 5 IDENTICAL SHELVES WOULD have to be 76 because if you take 400 - 20 (because that left over) you get 380/5 you get 76 and to check your work you do 5x76=380+20 you get 400 so 76 is you answer
