A dragon is given 630 ounces of magical potion to sustain her journey from Arlandia to Gargandua. While flying 4/7 of the distance, she drinks 2/9 of the potion. Suppose she continues to drink the magical potion at this rate for the remainder of her journey. How much potion remains when she reaches Gargandua?

Can someone help me with this question? I can't even start!
P.S. Answering this question will give you the highest credit!