math
posted by Lisa .
In a community of 416 people, each person owns a dog or a cat or both. If there are 316 dog owners and 280 cat owners, how many of the dog owners own no cat?
The choices are:
36
100
136
180
316
I think the answer is either 180 or 316, but I'm not sure.

Do it with Venn diagrams
draw two intersecting circles, label the overlap as x
This will represent the people owning both cats and dogs
label one circle D for dogs, the other C for cats
label the part of circle D without the overlap as 316x
label the part of the circle C without the overlap 280x
now 316x + x + 280x = 416
x = 180
so the dog owners owning "no cat" would be those in the D circle outside the overlap or 316x = 316180 = 136 
Thank you.

how do you round

First, if you have a question, it is much better to put it in as a separate post in <Post a New Question> rather than attaching it to a previous question, where it is more likely to be overlooked.
Even so, I searched Google under the key words "rounding number rules" to get these possible sources:
http://math.about.com/od/arithmetic/a/Rounding.htm
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http://en.wikipedia.org/wiki/Rounding
http://www.math.com/school/subject1/lessons/S1U1L3DP.html
In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search.
I hope this helps. Thanks for asking.