Answer:
Last year, Donald earned $10,000.
Explanation:
Let us assume the Donald's earnings last year = $ m
The income taxes = 25% of the salary
Now, It is given that the tax paid last year = $2,500
⇒ 25% of m = $2,500
Solving for the value of m, we get

or, m = $10,000
Hence, the last year earnings of Donald = $10,000