somehow i got stuck up in this problem im havin trouble?

Question

http://i181.photobucket.com/albums/x85/ctti_3/untitled-20.jpg

Answer 1

Profit = revenue - cost
so P(x) = 410x - 700 - x^2/40
and dP/dx = 410 - x/20.

We are given dx/dt = 4 when x = 120 and want dP/dt. The chain rule says dP/dt = dP/dx . dx/dt.
So dP/dt = (410 - x/20) (4) = 1640 - 120/5
= $1616 per week.