Using the most powerful telescope ever made, scientists observe a class of young aliens on a planet millions of light years away. On the blackboard, their teacher has written these equations: 13 + 15 = 31 10 * 10 = 100 6 * 3 = 24How many fingers do they have? (We count in base ten. What do they count in?)

1) 5
2) 6
3) 7
4) 8