Answer: seconds
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With conversion fractions, the denominator unit is always what you want to cancel out. In this case, we want to cancel out "seconds" because we want to effectively replace "seconds" with "minutes" (which is what conversions are really). The numerator is the unit we want to convert to. Take care to note that sometimes you'll need multiple conversions, and sometimes it's not going to involve one conversion fraction only.
Have a look at the attached image to get a better sense of what is going on. Note the red slash marks to indicate cancellations.
