How can I determine if x is directly proportional to y or not?
For,example we know that 2 atoms of Hydrogen(x) combine with 1 atom of Oxygen(y).
How can I establish here that x (number of Hydrogen atoms required) is proportional to y (number of Oxygen atoms)?
Wikipedia says that If one variable is always the product of the other and a constant, the two are said to be directly proportional. x and y are directly proportional if the ratio y/x is constant.
Constant can be any real number except 0. And for case of this oxygen and hydrogen ratio $y/x=1/2$