Question

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

Answer 1

It would look like this: 60/1 min = x/180 sec

Answer 2

you can look at it this way,

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

$\bf \begin{array}{ccll} seconds&minutes\\ \text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\ 60&1\\ 180&x \end{array}\implies \cfrac{60}{180}=\cfrac{1}{x}\implies x=\cfrac{180\cdot 1}{60}$

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.