In order to get the proper answer (the one your teacher expects) you are going to have to find out what exact definition your teacher is using for dependency ratio. Basically, you are trying to find out what the ratio of people who aren't of working age to the people who are of working age is. To do that, you need to know how your teacher is defining "working age."
Here's an example of how to do this. I will define dependents as those who are under 15 years of age or over 65. So we need to find the number of those people and divide it by the number of people who are 15-65. So now you need to go into those census numbers you provided and add up the number of people in each of the 0-15 categories (there are 3 categories you need to add). Then add the over 65s (there are 8 categories). Add those together and you have your dependent population.
Take the dependent population and subtract it from the total. Now you have your working age population. Divide the dependents by the working age and you have your ratio.