The average cost function for the weekly manufacture of portable gramophones is given by
$\bar{C}(x) = 140,000x^{-1} + 30 + 0.0004x$ dollars per gramophone,
where $x$ is the number of gramophones manufactured that week. Weekly production is currently 4,000 gramophones and is increasing at a rate of 400 gramophones per week. What is happening to the average cost? [Hint: See Example 3.] (Round your answer to the nearest cent.)
The average cost is decreasing at a rate of $\$$____ per gramophone per week.