Math Help - Relative rates

1. Relative rates

when the area in squre units of an expanding circle is increasing twice as fast as its radius in linear units,the radius is

a) 1/4pi
b)1/4
c)1/pi
d)1
E)pi

anyone know how to explain this i just learned Related Rates and not that good at it.

( o yea the titles suppose to be Related Rates)

2. Ok so let's start with our basic area formula for a circle.

$A= \pi r^2$

Now let's differentiate with respect to t, or time.

$\frac{dA}{dt} = 2 \pi r \frac{dr}{dt}$

Now the problem says something particular about the ratio of the changing area to the changing radius. Can you fill that in and solve for r at that moment?

3. i dont think i can