Explanation

Adding the numbers together won’t take long, but there is a shortcut to this problem.Match up the smallest number of miles (1) with the largest number of miles (12), and then the second smallest (2) with the second largest (11), etc. until all the numbers are matched up.

1 12
2 11
3 10
4 9
5 8
6 7


The sum of each of these 6 pairs is 13, which means you could multiply 13 by 6 = 78 to get the total number of miles. Next, find the prime factors of 78. We already know it’s divisible by 13 and 6 (because we had multiplied those together to get 78). Given that 13 is prime, you now need to find the prime factors of 6, which are 2 and 3. All together we have 3 prime factors at the bottom of the prime factor tree, so that’s our answer.





Hence 3 is the correct answer.