There are 20 students in Joey’s PE class who are participating in an extracurricular walkathon to raise money for a field trip. The students are divided into two groups and each group is walking in opposite directions from the other group. Joey is the fastest student in his group and Natasha is the fastest student in her group. At the end of 2 hours, Natasha is about 4 km ahead of Mark who is the second fastest person in her group. On average, Joey walks about 2 km/hr faster than Natasha. The students will earn $1.50 per kilometer. If Mark earns $33. How much more does Joey earn than Natasha?