Ok, work out the time first, FL320 (32,000 ft) to FL250 (25,000 ft) is 7000 ft. Divided by 800 ft per minute descent is 8.75 minutes. 540 mph is 9 miles a minute (540/60), so distance would be 9*8.75 = 78.75 miles.
Probably a little more complicated than you'd get in the motivation, although there was one a little similar. I don't think it needed you to know the conversion of FL to real altitude, but I may be wrong as it's just a natural thing for me now.