Rental car or train? It’s a question I face every time I head back to England, trying to figure out the best way to travel. And after doing a few sums, there was no contest.

Shares of Yahoo! Inc. (YHOO) tumbled 15% yesterday (Monday) to close at $24.37 a share as investors responded to Saturday’s news that Microsoft Corp. (MSFT) would drop its $47.5 billion dollar bid for the beleaguered search engine firm.

But the tough times are just beginning for Yahoo, which must now prove why it is worth more than the lofty price Microsoft was offering.