math
posted by ray .
A poll of 100 students was taken at a noresidential college to find out how they got to campus. The results were as follows: 28 said car pools;31 said buses;and 42 said they drove to school in their own car alone. In addition, 9 used both car pools and buses;10 said both car pools and using their own car; and 6 used buses and sometimes used their own car. Of the 100 respondents, only 4 used all three methods to get to school. Can you show me how to set up the problem and answer how many students used none of the three methods to get to school?

This is perfect for a Venn diagram. Draw three intersecting circles, where there is a nonempty space where all three circles intersect.
Start from the center and work your way out.
The center area contains 4.
9 used carpool and bus. Of those, 4 used all three methods, leaving 5 inside the carpoolbus intersection, but outside the center.
10 used carpool and owncar. Of those, 4 used all three methods, leaving 6 inside the carpoolself intersection, but outside the center.
That makes 4+5+6=15 who used carpool and some other method. So, that leaves 13 who used ONLY carpool.
Do similar figuring for only bus and only selfcar.
Add up the numbers in all the sections of the diagram, and subtract that from the Universe of 100 students. Those will be the ones who used none of the methods.