what is variation and there method

I was trying to finds a good page for you but I couldn't find one that I was completely happy with

basically if two things have direct variation then as one gets bigger so does the other - and vise versa

the example in the first web page is distance traveled and time taken.

time taken is directly proportional to distance travelled because 

the longer the travel time the futher you travel.

Have a look at these web pages if you want more into.    

http://www.mathsteacher.com.au/year10/ch17_variation/05_directvariation/25direct.htm

This is more invoved:

https://www.khanacademy.org/math/algebra/algebra-functions/direct_inverse_variation/v/direct-and-inverse-variation