Let's define some notation first:
H= number of Hogwarts, R= number of robes, Q= number of Quidditch
We can set up the following equation for the total of clothing items:

We also know that Q= 2R so we have:

Now we can create an equation for the amount of money and we have:


Now we can replace H=200-3R and we got:

And solving for R we got:


And then Q would be:
