What is the formula for calculating the useful heart rate?

The useful heart rate formula is used to monitor the intensity and progression of physical exercise.
The formula includes the resting heart rate and the maximum heart rate.
The duration of each exercise session should be at least 30 minutes.
The useful heart rate formula is not useful for individuals with cardiovascular disease.
a) The first and second statements are correct.
b) The second and third statements are correct.
c) The first and fourth statements are correct.
d) The second and fourth statements are correct.