A doctor observes a graph that shows the electrical activity (in volts) of the heart of a patient over a period of time (in seconds). Each local maximum of $0.004$ volts corresponds to one heartbeat. The doctor needs to calculate the heart rate of the patient in *beats per minute*. What is the heart rate of this patient?
[[☃ math-keypad 1]] beats per minute
[[☃ image 1]]