There are proponents of a new type of computing that uses strands of DNA in a test tube to perform computations–to solve problems like the "Traveling Salesman" problem. These proponents note that in a small volume, there are trillions of small DNA chunks that could (in effect) try trillions of different tours for the salesman, and report the shortest tour. Here's your question: Suppose the salesman has to visit N cities. How big does N have to be before the number of tours that he would have to test (in the worst case) is larger than a trillion?