It doesn't look like it can be cleared out from a problem description. If so, does the speed of a cart remains equal 'v' after tunnel exiting, or the same, as at the end of a side tunnel?

NO
"Meanwhile, the wall of fire continued to approach the cart. Time was running out, and a plan had to be developed fast. WIZARDS CAN TURN TO A SIDE TUNNEL ONLY ONCE BECAUSE THE CART IS VERY OLD AND NOT SUITED WELL FOR MAKING TURNS. But which tunnel to use?"

Edited by author 21.10.2012 12:13