Answer:
His resting heart rate might decrease to 60 bpm
Step-by-step explanation:
The average resting heart rate range is 60 bpm - 100 bpm. When people are healthier they tend to have lower resting heart rates. Many athletes have a resting heart rate of 60 bpm.
Training for a 10k race would make Charlie's heart work more during training, thus it doesn't beat as often during rest.