>>73
線形なら、下記だけど。
> predict(lm(y ~ x, data = data.frame(y = c(2488, 2986), x = c(15,20))), data.frame(x=c(25, 28, 30)))
1 2 3
3484.0 3782.8 3982.0

25pt: 3484
28pt: 3782
30pt: 3982