A lake near the Arctic Circle is covered by a sheet of ice during the cold winter months. When spring arrives, the ice starts to melt.
$S(t)$ models the ice sheet's thickness (in meters) as a function of time $t$ (in weeks).
$S(t)=-0.25t+4$
**By how much does the sheet's thickness decrease every $6$ weeks?**
[[☃ math-keypad 2]] meters