derivatives question - please help !?

Question

Can anyone walk me with the follow problem please?

A company can manufacture x items at the cost of c(x) dollars, taken r(x) on these items and realize a profit p(x) = r(x) - c(x) dollars (all amounts in thousands)
Find dc/dt, dr/dt, and dp/dt for the following values of x and dx/dt:

a. r(x) = 70x, c(x) = x^3 - 6x^2 + 45/x and dx/dt = .05 when x=1.5

Answer 1

Use the chain rule:

dF/dt = (dF/dx)(dx/dt)

http://en.wikipedia.org/wiki/Chain_rule

For example:
dr/dt = (dr/dx)(dx/dt) = (70)(.05) = 3.5