$3.(20$ points$)$ Evaluate the S$-$curve $1(1$ tr$)$b for similarity value t $=0\mathrm{.}1,0\mathrm{.}2,0\mathrm{.}3,0\mathrm{.}4,0\mathrm{.}5,0\mathrm{.}6.0\mathrm{.}7,0\mathrm{.}8,0\mathrm{.}9,$ for the following values of r and b:

a$.$ r $=10$ and b $=20$

b$.$ r $=5$ and b $=15$

c$.$ r $=25$ and b $=10$

$4.(20$ points$)$ For each of the $($r$,$ b$)$ pairs in question $3,$ compute the threshold, that is$,$ the value of s for which the value of $1(1$ tr$)$b is exactly $3/4.$