To covert 180 seconds to minutes, which proportion should you use?

You can look at it this way,

there are 60 seconds in 1 minute, how many minutes in 180 seconds?



but usually, the proportion wil be a fraction, so usually you'd put the unit you need cancelled at the bottom or the denominator.

60s/m  or m/60s , in this case we need to cancel the seconds, so 180 s * m/60s.