Justin"s heart rate decreased 3 beats per minute for each of next 4 minutes
Let his heart rate be be 1
For the first minutes
1 minute his heart rate was 1 - 3 = -2
This is because the heart rate is decreasing 3 beats per minutes
2minutes later, his heart beat will be -2 -3
-2 - 3 = -5
3 minutes later, his heart beat will be -5 - 3
-5 - 3 = -8
4 minutes later, -8 - 3
-8 - 3 = -11
Change = Final heart beat - initial heart beat / initial heart beat
Change = -11 -2 / -2
Change = -13/-2
Change = 13/2
= 6.5
