Question

When I do (floor 4 3) I got

1
1/3

But how do I use that 1/3?

Answer 1

You can for instance bind it to a variable using multiple-value-bind.

(multiple-value-bind (quot rem)
    (floor 4 3)
  (format t "The remainder is ~f~%" rem))

Another possibility, if you’re only interested in one non-primary value, is nth-value.

(format t "The remainder is also ~f~%" (nth-value 1 (floor 4 3)))

For reference, see the Hyperspec.