Final answer:
The remaining value in dollars of the gift card after 20 days is $25.
Step-by-step explanation:
To find the remaining value in dollars of the gift card after 20 days, we need to substitute the value of n as 20 into the function f(n) = -2.5n + 75.
So, f(20) = -2.5 * 20 + 75
= -50 + 75
= 25.
Therefore, the remaining value in dollars of the gift card after 20 days is $25.