Sunday Afternoon Maths XIX

Here's this week's collection. Answers & extensions tomorrow. Why not discuss the problems on Twitter using #SundayAfternoonMaths or on Reddit.

Dartboard

Concentric circles with radii 1, $\frac{1}{2}$, $\frac{1}{3}$, $\frac{1}{4}$, ... are drawn. Alternate donut-shaped regions are shaded.

What is the total shaded area?

Multiples of Three

If the digits of a number add up to a multiple of three, then the number is a multiple of three. Therefore if a two digit number, $AB$ (first digit $A$, second digit $B$; not $A\times B$), is a multiple of three, then $A0B$ is also a multiple of three.

If $AB\div 3=n$, then what is $A0B\div 3$?