Calculas related rates question

Lumber in a spruce forest in valued at $1 million per square kilometer. If the radius of a circular shaped forest fire is 4 km and is incresing at the rate of 0.1km/h, find the rate at which money is being lost

I tried to solve this by using the area formula but I dont know where to sub in that 0.1km/h

Value = V
Area = A
dV/dA = 1 mil./sq. km.
Radius, r = 4 km.
dr/dt = 0.1 km./hr.

Find dV/dt:

A = π r²
A = (4)²π
A = 16π
A = 50.27 sq. km.

Differentiate Implicitly Over Time:

dA/dt = 2πr(dr/dt)
dA/dt = 2π(4)( 0.1)
dA/dt = 8π(0.1)
dA/dt = 0.8π
dA/dt = 2.5 sq. km./hr.

dV/dt = (dA/dt)(dV/dA)
dV/dt = 2.5(1)
dV/dt = 2.5

The loss is 2.5 million dollars/ hr.
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A = pi*r^2

dA/dt = 2*pi*r(dr/dt)

dA/dt = 2*pi*(4 km)(0.1 km/h) = 0.8*pi*km²/h

Rate at which money is being lost = 0.8*pi*km²/h * $1M/km² = $800,000pi/h

A = pi * r^2
dA/dt = 2 * pi * r * drdt

r = 4
dr/dt = 0.1

dA/dt = 2 * pi * 4 * 0.1 = 0.8 * pi

Multiplied by 1 million dollars:

800,000 * pi dollars is being lost

Your .1km is your dR/dT.
A=piR^2
A'=2piR(dR/dT)
A'=2pi*4*.1
A'=2.513