Most of the energy used during exercise comes from aerobic respiration - Edexcel - GCSE Biology - Question 4 - 2018 - Paper 1

To calculate the heart rate, use the formula:

$ext{Cardiac Output} = ext{Stroke Volume} imes ext{Heart Rate}$

Here, we know the stroke volume during gentle exercise is 0.13 dm³ and the required cardiac output is 15.6 dm³ per minute (given):

$15.6 = 0.13 imes ext{Heart Rate}$

Solving for Heart Rate:

$ext{Heart Rate} = \frac{15.6}{0.13}$

$ext{Heart Rate} = 120 ext{ beats per minute}$

Thus, the heart rate of the athlete during gentle exercise is 120 beats per minute.