Answer:
They never collide.
Step-by-step explanation:
We have the equations:

If the two girsl collide at a certain t, then

Then, we can equal each pair of euqations to find if there is any time t when their positions are the same.
Then:

And solve for t:

Now, if the girls collide, at t = 21s they will be at the same place in in y(t)
Let's see:

And solve:

Now, for Allison:

And solve:

Then, we have:
At t = 21s the two girls are at the same position in the x axis. But, at 21s, in the y axis, Amy is at 9.5 and Allison at 10.25
To collide, they have to be in the same position in the x and y axis at the same time.
Thus, they never collide.