I worked this problem and found some strange answers: 2 thin lenses separated by 10cm. First has a focal length of 20cm, the second has a focal length of 30cm. Here are my answers:

Where H denotes the principle planes and O denotes the lenses.

So here's the problem, recall that the lenses are separated by only 10cm, this forces principle plane #2 to be in front (left of) principle plane #1. How can this be true? What did i do wrong?