Answer:
12 beats per minute after 30minutes
Explanation:
Since we are told by how much his heartrate increased in 20 minutes, we can simply calculate how much it increased in 10 minutes and add that amount to the current heart rate to know how much his heartrate increased after 30 minutes of working out. Since 10minutes would be half of the 20minutes we simply divide the beats per minute by 2.
8 / 2 = 4 beats per minute after 10 minutes
Now we simply add this to the beats per minute at the 20minute mark to calculate the beats per minute at 30minutes
8 + 4 = 12 beats per minute after 30minutes