5pi/6
There are 60 minutes in one hour. In a clock this means half of it is 30 minutes.
There are 2pi radians on a complete unit circle. This means half of the unit circle is 1pi.
It is asking for 25 minutes and this can be broken up into 5 minute intervals. This means there are six 5 minute intervals in 30 minutes.
When pi is broken up into 6 pieces you simply divide it by 6 giving you; pi/6
This means that in 30 mins a minute hand would move pi/6 radians, and in 25 minutes it would move 5pi/6 radians.