An investor bought a one-acre lot on the outskirts of a city for $9000 cash. Each year she paid $80 of property taxes. At the end of 4 years, she sold the lot for a net value of $15,000. What rate of return did she receive on her investment?

Year Cashflow

0 -9000

1 - 80

2 - 80

3 - 80

4 - 80 (+ 15,000)

To solve for Rate of Return (ROR) I used :

PW_i = $-9,000 - $80 (P/A,i,4) + $15,000 (P/F,i,4)

@ 12% = $-9,000 - $80 (3.3037) + $15,000 (0.6355) = $289.54

@ 15% = $-9,000 - $80 (2.855) + $15,000 (0.5718) = $-651.40

If these are correct, please show me how to interpolate here between 12% and 15%. (Do not use excel)