If you think impurity is eluting at  25 minutes, than relative retention time (RRT) with respect to your major peak would be 25/20 = 1.25 

Thank you sir 

Relative retention time (RRT) is the ratio of the retention time of analyte peak relative to that of another used as a reference obtained under identical conditions.

RRT = (Tanalyte / T reference)            Where T = Retention time

There is no acceptance criteria applied for RRT

RRT is to be measured with unadjusted and identical condition

The relative retention time (RRT) reduces the effects of some of the variables that can affect the retention time. RRT is an expression of a sample’s retention time relative to the standard’s retention time.

RRT = Standard RT / Sample RT

Relative retention time (RRT) is the ratio of the retention time of any extra peak in relative to that of principle peak reference obtained in same chromatography run under identical conditions.

RRT= RT of required peak/RT of principle

Relative retention time = Retention time for analyte/ Retention time for internal standard

Criteria:

2002/657 EC directive; RRT deviation