Duration * Reps will calculate a total duration in seconds. If you select a time result type, you can format the field on your layout to get a time format.
but 1:30 is notation for 1 hour, 30 minutes, not 1 minute, 30 seconds. That may not be a major issue since you can use data formatting in the Inspector to use 24 hour format and the result will have the correct appearance even though it has computed the wrong value.
Otherwise, you'll need to change either your data input or set up a calculation that first converts 1:30 into minutes and seconds before computing the elapsed time.