好
用插值法
100-100*5%*(P/F,K,8)=20*(P/A,K,8)
100*5%*(P/F,K,8)+20*(P/A,K,8)=100
当K=12%时,100*5%*(P/F,12%,8)+20*(P/A,12%,8)=101.37
当K=13%时,100*5%*(P/F,11%,8)+20*(P/A,11%,8)=97.86
所以
(12%-13%)/(12%-K)=(101.37-97.86)/(101.37-100)
(-1%)/(12%-K)=3.51/1.37
12%-K=1.37/3.51*(-1%)
k=12%+0.4%
k=12.4%