I know there's no equations given but maybe this could work.
Possible answer: (25,000 - 12,100) / 50 = d
25,000 (end amount of miles)
subtracted by 12,100 (before added miles)
This will give you the amount the worker drove.
Since there's no known amount of days the worker has delivered, you want to divide.
The difference of the two would be divided by the 50 miles to find the amount of days.
So I think the best equation would be
(25,000 - 12,100) / 50 = d