How to know what units for pressure to use?

[Daniel Zhang 1E]

For question 4 on the 2011 practice midterm, the answer was given in bar even though there was no indication of it being used in the constants provided. How do we know to use bar rather than kPa or atm?

[PriyaKohli3B]

We do not, we can use any unit for pressure I would use bar or atm but I do not think it matters since units for pressure cancel out anyways.

[Daniel Zhang 1E]

But the units don't cancel out? It's