How many radians are there between the hands of the clock at eight o'clock?
In this digital age you probably aren't familar with an analogue clock so your difficulty is quite understandable.
There are twelve evenly spaced numbers from 1 to 12. There is a big hand and a little hand. See analog clock - Google Image Search
At 8 o'clock the big hand points to the 12 and the little hand points to the 8.
So the hands subtend 1/3 of 360 degrees, that is, 1/3 of $\displaystyle 2 pi$ radians ......
Suppose minute hand is fixed on 12 already. Now the hr hand can swap an angle of 2pi radians for a complete rotation.(assume that the hr hand is initially on 12)
for 12 hr-->2pi radians therfor 1 hr-->1/6pi radians
so in 1 hr or at 1 o'clock(as we assumed both hands on 12 initially) angle of 1/6 pi radian is swaped.so for 4 o'clock the angle will be 4/6 pi radians=2/3 pi radians.
From symmetry the angle between hr hand and min hand at 4o'clock is same as the angle between hr hand and min hand at 8o'clock
therfor for 8o'clock the angles between clock hands will be 2/3 pi radians