E16-5(Conversion of Bonds) The December 31, 2007, balance sheet of Kepler Corp. is as follows.

10% callable, convertible bonds payable (semiannual interest dates April 30 and October 31; convertible into 6 shares of $25 par value common stock per $1,000 of bond principal; maturity date April 30, 2013) $500,000
Discount on bonds payable 10,240 $489,760

On March 5, 2008, Kepler Corp. called all of the bonds as of April 30 for the principal plus interest through April 30. By April 30 all bondholders had exercised their conversion to common stock as of the interest payment date. Consequently, on April 30, Kepler Corp. paid the semiannual interest and issued shares of common stock for the bonds. The discount is amortized on a straight-line basis. Kepler uses the book value method.

The answer key is interest expense 25640
discount on bonds payable 640
cash 25000

I want to know how to get the $640. The answer key shows 102460/64=160, 160*4=640. Please help.