Your answer is correct. Indeed:
- In order to find the degrees per minute, you have to divide the full rotation angle (which is 360deg) by the number of minutes (which is 60):
360deg ÷ 60min = 6 deg/min
- In order to find how many minutes have passed, you subtract the times:
1h50m - 1h25m = 0h25m
- You then find how many degrees correspond to 25minutes:
6 deg/min × 25 min = 150deg
- Lastly, you transform degrees into radiants, through the proportion:
deg : 180 = rad : π
Therefore,
150 : 180 = rad : π
rad = (150×π)/180 = 5π/6