Answer:
A small slushee has 12 ounces.
A medium 29 ounces.
And a large 72 ounces.
Explanation:
Let s represent the size of the small slushee, m the medium slushee, and l the large slushee.
We know that the medium slushee has 17 more ounces than the small slushee. So:

We also know that the large slushee l has 6 times as many ounces than the small slushee. Therefore:

Finally, we know that there are a total of 113 ounces in the small, medium, and large drinks combined. So:

We can solve this triple system of equations using substitution. Let's substitute l for 6s and m for (s+17). This yields:

Combine like terms:

Subtract 17 from both sides:

Divide both sides by 8:

So, the small slushee is 12 ounces.
So, the medium slushee is

And the large slushee is

And we're done!