For this kind of thing I generally use the remainder function and multiply the originally number out before I go after the remainder.
Failing that being available, I use a formula that takes the first number and subtracts a 'fix' or 'int' of the second number. Fix is a function usually available that returns the number without the decimal (int is sometimes exactly the same). Look through your help file for each of these.
It will look like ( {numberfield}-fix({numberfield}) ) * 60 to get the seconds.
So in your example the 15.75 would first become .75 and then multiply by 60 to give 45.
As I recall, the difference between Int and Fix had to do with negative numbers, where INT would give you the next lower number, and FIX would give you the next higher number.
So INT(-4.5) = -5
FIX(-4.5) = -4
but your help file will let you know. Since you aren't likely to have negative seconds the difference may not matter.
Oh, you did ask if this could be done in one formula. Yes, although it is easier to follow if you do it in two. If you had to do it in one formula you would have to turn the numbers into text:
"Minutes: " + totext(fix({numberfield)) + " " + "Seconds: " + totext(( {numberfield}-fix({numberfield}) ) * 60)
Although I wouldn't go to that trouble. I'd make a formula for minutes, another for seconds, plunk a text field box onto the report and drag the two formulas down, then put the text and stuff around them.
Scott.