I would have to disagree.

Using Sir Wilmer's highly enviable and clever (and may I say ingenious) formula (simplified) for determining the amount of interest paid between payment x = 56 and payment y = 68 (interest paid from 57th/January1993 up until 68th/December1993), I get

where

and

Your given answer is really more like interest paid between payment x = 55 and payment y = 67 (interest paid from 56th/December1992 up until 67th/November1993). Thus

For more of Sir Wilmer's/Denis' original expanded formula, go here.

See my Excel spreadsheet proof.