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

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

Step 1 ：Suppose that Marissa bought a new tablet for $400.Theresalevalueisexpectedtodecreaseby$40 every year for the next several years.

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

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

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