
A ratio is a a association or a corelation between two numbers. It is represented using colon or fractions, as a:b or $\frac{a}{b}$. If two ratios are equivalent then we write it has $\frac{a}{b}$ = $\frac{c}{d}$ proportion or balance is a relative relation between things or magnitudes such as size, quantity etc. Example: x : 3 :: 9 : 6 Here, the first set of ratios is equal to second set of ratios, thus we say that ratios are in proportion. Proportion Calculator is used to calculate the unknown quantity of a set of ratios. 
Observe the given two ratios.
Do the crossmultiplication and then isolate the unknown variable.
A bag contains balls of color blue and black, if the ratio blue balls to black balls is 4:8 and contains 230 black balls, then find the total number of blue balls in the bag?
Given that:
Total number of black balls = 230
Let total number of blue balls = x
Given ratio $\Rightarrow$ $\frac{Blue}{Black}$ = $\frac{4}{8}$ = $\frac{x}{230}$
$\frac{4}{8}$ = $\frac{x}{230}$
After crossmultiplication
4 $\times$ 230 = x $\times$ 8
920 = 8x
Divide by 8 on both sides
x = 115
So, total number of blue balls = 115
Find the value of x in $\frac{2}{9}$ = $\frac{5}{x}$?
Given ratios
$\frac{2}{9}$ = $\frac{5}{x}$
After crossmultiplication
2 $\times$ x = 5 $\times$ 9
2x = 45
Divide by 2 on both sides
x = 22.5
x = 22.5