Related Rates Question: Rate of Money Lost

[grapz]

Lumber in a spruce forest is valued at 1 mil / km^2. if the radius of a circular-shapped forest fire is 4km, and radius of area that is covered by the fire is increasing at a rate of 0.1 km/h , find rate at which $ is beign lost

I can't find a relation for this problem... please help

i mean i understand there is one but i just can't grasp it

 

[galactus]

Area of circular region is given by \(\displaystyle \L\\A={\pi}r^{2}\)

Differentiate with respect to time:

\(\displaystyle \L\\\frac{dA}{dt}=2{\pi}r\frac{dr}{dt}\)

Can you finish knowing how much the forest cost per square km?.