The wind$-$chill index is modeled by the function

W $=13\mathrm{.}12+0\mathrm{.}6215$T $11\mathrm{.}37$v$0\mathrm{.}16+0\mathrm{.}3965$Tv$0\mathrm{.}16$

where T is the temperature $($C$)$ and v is the wind speed $($in km$/$h$).$ When

T $=12$C

and

v $=20$ km$/$h$,$

by how much $($in $$C$)$ would you expect the apparent temperature W to drop if the actual temperature decreases by $1$C$?($Round your answers to two decimal places.$)$