F2 IRR

Hi

I'm currently doing F2 and have an exemption from F1 so I'm a bit rusty.

When doing a IRR calculation can you pick any random % rates as long as one is plus and one is minus, it's just I have picked different rates from the answer so I am coming up with a different rate.  When I re-calculate using the same rates the answer has I get it right but just wondering how that would work in an objective test.

Thanks