Related rates circle problem Two circles $A$ and $B$ have the same center. The radius of the inner circle $A$ is increasing at a rate of $1$ unit/sec, and the radius of the larger circle $B$ is also increasing such that the area between the two circles is always $10\pi$. When the radius of $A$ is 5, how fast is the radius of $B$ increasing?
I know I need to start by setting up an equation to perform related rates, but what that equation needs to be and how I need to solve it, I don't know (Note: I understand the topic, this is not that kind of issue).

Let $R, r$ be the radii of the larger and smaller circles, respectively. Then:
$$\pi R^2-\pi r^2=10 \pi \Rightarrow R^2-r^2=10.$$
Differentiate with respect to time $t$:
$$2RR'-2rr'=0 \Rightarrow R'=\frac{rr'}{R}=\frac{5\cdot 1}{\sqrt{10+5^2}}=\frac{\sqrt{35}}{7}. $$
