25d
or we can also write m for money, a formula for that, actually a function: M=25d
1) Gathering the data from the question
$25/ day
How much money in D days?
2)
1 day $ 25
2 50
3 75
D 25D
So, Trevor will earn 25D where D is the number of days He's worked.
Plug the number of Days and multiply by 25
M=25d