dublinpilot has it right!
It's an exam question.
So...
ATFQ!
It doesn't matter what we would do in real life, the answer (rounded up to the nearest FPM) is 249fpm. Personally, I start a descent when I feel like it. Usually at a convenient position report. I can then use a cruise descent, or faster if necessary. I can't recall when I last used the mathematics involved in answering this question in real life.