2/5 - PR for 5 years then 3/5?
3/10?
Say you became PR on January 1, 2020. If you stay a full 2 years without leaving the country, you've met your residency obligations, so you can renew your PR when your PR card expires**.
If you stay for another year (total 3 years), you can apply for citizenship on January 1, 2023. The way it works is that they'll look at how many years you were a resident of Canada in the previous 5 years, starting from January 1, 2023 (date you submitted). So between 1/1/18 and 1/1/23.
**If you stay exactly 2 years then leave, they will probably make it a lot harder for you to renew your PR card, as you'll need to show evidence that you intend to live in Canada. That's what I've been reading on this forum anyway.