# Task 3. (3 points)

In the country of Taxia, everyone pays as many thousandths of their salary in taxes as tugriks the salary amounts to. What salary is the most advantageous to have?

(Salary is measured in a positive, not necessarily integer number of tugriks)
Answer: 500 tugriks

Solution:

Let the salary be $x$. Then what remains after the tax deduction is $x-\frac{x^{2}}{1000}$. This is a quadratic trinomial with a negative leading coefficient, it reaches its maximum at the vertex when $x=500$.