we know that
[length of a circumference ]=2*pi*r
for r=1 units
[length of a circumference ]=2*pi*1------> 2*pi units
if 2*pi radians (full circle) has a length of-----------------> 2*pi units
pi/3 radians-----------------------------> X
X=(pi/3)*2*pi/(2*pi)-----> pi/3 units
the answer is
pi/3