To find a missing numerator or denominator of a fraction, another fraction of equal proportion must also be present so that a ratio can be set up and solved for the missing value. Ratio is just another term for a fraction, and ratios are used in the real world for things like doubling a cooking recipe.
Ratio problems are solved by setting the two fractions equal to one another. After that, the fractions are cross-multiplied and the variable isolated. For example, in 2/3=x/6, the cross-multiplication gives 12=3x. The variable is then isolated by dividing both sides by 3 to get x=4.