1. The problem states that the thickness of the ice sheet $s$ (in meters) depends on the number of weeks $t$ after the beginning of spring, modeled by the equation $$s = -0.25t + 4.$$ 2. To find how much the ice sheet's thickness decreases every 6 weeks, we compute the change in thickness when $t$ increases by 6. 3. Calculate the thickness at $t=0$: $$s_0 = -0.25(0) + 4 = 4.$$ 4. Calculate the thickness at $t=6$: $$s_6 = -0.25(6) + 4 = -1.5 + 4 = 2.5.$$ 5. The decrease in thickness over 6 weeks is: $$ ext{decrease} = s_0 - s_6 = 4 - 2.5 = 1.5.$$ 6. Therefore, the ice sheet's thickness decreases by 1.5 meters every 6 weeks.