Answer:
- One of the donors offers to double the total donation amount at the end of the night.
Explanation:
Consider the answers one by one.
Doubling the total donation amount will result in Percy's D(x) function becoming 2D(x), a vertical dilation by a factor of 2. (This first answer is what you're looking for.)
Half the donors getting sick and not showing up likely results in a donation amount of D(x/2), a horizontal scaling.
Postponing the event for 2 days does not change Percy's D(x) function. If the function is time-sensitive, as in D(x, t) or D(x(t)), then there will be some change, but it is probably not vertical dilation.
If twice as many people come, the donation amount may be D(2x). As with D(x/2), this is a horizontal scaling, not a vertical one.