Suppose that Marissa bought a new tablet for $\$ 400$. The resale value is expected to decrease by $\$ 40$ every year for the next several years.

Define a formula that determines the change in the value of the tablet computer, $\Delta v$, in terms of the change in the number of years, $\Delta t$, since she bought it.
$\Delta t=400+40 \Delta t$
$\Delta t=-40 \Delta v$
$\Delta v=-40 \Delta t$
$\Delta v=40 \Delta t$
$\Delta v=400-40 \Delta t$

Step 1 ：Suppose that Marissa bought a new tablet for $400. The resale value is expected to decrease by $40 every year for the next several years.

Step 2 ：Define a formula that determines the change in the value of the tablet computer, \(\Delta v\), in terms of the change in the number of years, \(\Delta t\), since she bought it.

Step 3 ：The value of the tablet decreases by $40 every year. Therefore, the change in value, \(\Delta v\), is equal to the initial value minus the decrease per year times the number of years, \(\Delta t\).

Step 4 ：The formula that determines the change in the value of the tablet computer, \(\Delta v\), in terms of the change in the number of years, \(\Delta t\), since she bought it is \(\boxed{\Delta v = 400 - 40 \Delta t}\).