An experiment to measure the toxicity of formaldehyde yielded the data in the table below. The values show the percent, $P=f(t,c)$, of rats surviving an exposure to formaldehyde at a concentration of $c$ (in parts per million, ppm) after $t$ months.

 $t=14$ $t=16$ $t=18$ $t=20$ $t=22$ $t=24$ $c=0$ 100 100 100 99 97 95 $c=2$ 100 99 98 97 95 92 $c=6$ 96 95 93 90 86 80 $c=15$ 96 93 82 70 58 36

(a) Estimate $f_t(18,6)$:
$f_t(18, 6)\approx$

(b) Estimate $f_c(18,6)$:
$f_c(18, 6)\approx$

(Be sure that you can give the practical meaning of these two values in terms of formaldehyde toxicity.)