If you have a series of values like this:
12.5
9.3
10.7
and want to convert this numbers to a minutes/seconds format you can use this formula:
=A1/(24*60)
This assumes that the values below are on cells A1:A3 and this formula is placed on cell B1 and copied down. You need to format the cells B1:B3 as Custom mm:ss to see this:
12:30
09:18
10:42