Yep, your PVs of Project A at 18% and at 20% are good.

And you correctly reasoned that the actual IRR must lie between 18 and 20, hence your approximation via linear interpolation.

Just for grins, though, keep in mind that the relationship between PV and IRR isn't linear, and so a linear interpolation might miss it by more than your desired accuracy degree. Figure the PV at 19%, and then interpolate using 18 and 19% as your range.

It comes down to how much accuracy is appropriate for the situation--your call. But with the 18-19 interpolation, you'll have an idea of how sensitive the answer is to further fine-tuning. If the answer didn't move much, you'll probably be comfortable calling it a day without any further trial-and-error.

Cheers!

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