Crystal is 12 years old and alexa is 6 years old. if crystal runs 100 yards, how far does alexa run? how far do they run altogether?

Here we do not have enough information; let's suppose that the age is directly related to the distance that the girls can run:

We know that Cryistal has twice the age of Alexa (or Alexa has half the age of Cryistal)

Then is safe to assume that Alexa runs half as much as Cryistal does:

Then if Crystal runs 100 yards, Alexa runs half of that, and the half of 100 yards is 100/2 yards = 50 yards

So Alexa runs 50 yards

And for the distance that they run altogheter we should add both distances; this is:

100 yards + 50yards = 150 yards