Target heart rate (220- age* 50 to 85) if I'm 15, what is my target heart rate?

To calculate your target heart rate, use the formula:

Target Heart Rate = (220 - Age) * Intensity

For your age of 15, let's assume you want to exercise at a moderate intensity (around 65-75% of your maximum heart rate). Using an intensity factor of 0.65:

Target Heart Rate = (220 - 15) * 0.65

Target Heart Rate = 205 * 0.65

Target Heart Rate = 133.25

So, your target heart rate would be approximately 133 beats per minute.