This is really just another version of the old puzzle which uses MPH instead of MPG to ask essentially the same question: if I averaged 25MPH on the way there, and want to average 50MPH for the whole journey, how fast do I need to go on the way back? (Like your question, the answer is actually independent of the distance.)

If you use "litres per 100km", like in continental Europe, the figures do average linearly as per hybrid8's reply.

Peter