In circle A, the length of arc BC is 20π units.

The circumference of a circle is   pi x diameter

 

the arc length shown is    150/360 = 20 pi

 

then the CIRCUMFERENCE is   20 pi x  360/150 = 48 pi       48 pi/ pi = diameter = 48   Radius = 1/2 dia = 24

check

 

\(\frac{150}{360}*2*\pi*24 \\= \frac{5}{12}*2*\pi*24 \\= \frac{5}{1}*2*\pi*2 \\=20\pi\;\; units\)

 

GREAT

 

ElectricPavlov's answer is spot on  :)