Final answer:
To find the heart rate, use the formula: Heart Rate = (Number of beats / Time) x 60.
Step-by-step explanation:
To find your heart rate, you need to know how many beats occur in a given time period. In this case, you counted 18 beats in 15 seconds. To find the heart rate, you can use the formula:
Heart Rate = (Number of beats / Time) x 60
Plugging in the values, we get:
Heart Rate = (18 beats / 15 seconds) x 60 = 72 beats per minute