Theory:

When two ratios are in proportion \(a:b::c:d\), then the product of the extremes is equal to the product of the means. This is called the proportionality law.
 
Here '\(a\)' and '\(d\)' are the extreme values and '\(b\)' and '\(c\)' are the mean values.
 
Also, if two ratios are equal, then they can be related as follows.
 
ab=cdad=bc
 
It is called the cross product of proportion.
Example:
Let us check the ratios \(7:2\) and \(21:6\) are in proportion.
 
We can use the proportionality law to check the condition.
It says that 'When two ratios are in proportion \(a:b::c:d\), then the product of the extremes is equal to the product of the means'.
ab=cdad=bc
 
Here \(a = 7\), \(b = 2\), \(c = 21\) and \(d = 6\).
 
Apply the values in \(ad = bc\).
 
\(7×6 = 2×21\)
 
\(42 = 42\).
 
As the proportionality law satisfies, the given ratios are in proportion.