Of the 90 people on William's bus, 3/5 were Dutch. Of the 1/2 of the Dutch who were also American, 1/3 got window seats. What's the number of Dutch Americans who sat at the windows? Think carefully first, then make a decision:
On the bus, the number of Dutch people was 3 / 5 of the total number, a total of 3 / 5 * 90 = 54 people. Out of the 54 people who were Dutch, 1 / 2 were Dutch Americans, a total of 1 / 2 * 54 = 27 people. If 1 / 3 of the passengers on the bus identifying as Dutch Americans sat at the windows, their number is 1 / 3 * 27 = 9. So, the answer is 9.