Answer is 25 m/s.
Explanation;
Step 1 : Convert the units into same unit.
Here speeds are given in m/s while the time is in minutes. So, let's convert minutes into seconds.
1 min = 60 s
Then,
10 minutes = 10 x 60 s = 600 s
Step 2 : Find the total distance that you have travelled.
To find that you need to first find distances separately.
We know the formula, s = vt, where s is the distance, v is the speed or velocity and t is the time.
- First you have travelled 10 minutes (600 s) at 20.0 m/s.
Then distance, s = 20.0 m/s x 600 s = 12 000 m
- Secondly you have travelled again 10 minutes at 30.0 m/s
Then distance = 30.0 m/s x 600 s = 18 000 m
Then, the total distance = 12 000 m + 18 000 m = 30 000 m
Step 3 : Find the total time taken to travel.
Total time = 600 s + 600 s = 1200 s
Step 4 : Calculate the average speed.
Average speed = total distance / total time
= 30 000 m / 1200 s
= 25 m/s