Your middle school has 900 students. 1/3 of students bring their lunch instead of buying lunch at school. what is the value of the ratio of the number of students who do bring their lunch to the number of students who do not?

Hey there! 
This may seem pretty scary because, duh, fractions, big numbers, MATH.
However, you've just got to break it down.
So first, we find out how many students BRING lunch.
To find 1/3 of 900, you just multiply 900 by 1/3.  
BUT WE CAN SIMPLIFY: to 300/1 times 1/1, which is 300/1.
So 300 people BRING lunch.
Now, to find people who BUY lunch, you just subtract the number of people who bring lunch by the total number of people.
900
-300
600 
So 600 people BUY lunch.
All that's left to do is turn all these numbers into a ratio.
Easy peasy: put 300 and 600 like a fraction, like so:

Hope this helped! ;)