The same as you calculate the fraction of anything. For example, a fifth of an hour is one part out of five equal parts of a whole hour.
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-- One hour = 60 minutes. -- A fraction of an hour is the same fraction of 60 minutes. -- To convert a fraction of an hour to minutes, multiply the fraction by 60 minutes.
615 is a time of day: it is not, and cannot be expressed, as a fraction of an hour.
The fraction is 1.
It isn't clear what you want to calculate. Add fractions? Multiply them? Convert fraction to decimal? Decimal to fraction? The details of the calculations, of course, vary depending on what you want to calculate.
5 twelfths of an hour