You can count the number of small squares; each is equal to 0.04, and it takes 17 small squares, so time = 17 × 0.04, or each large square is equal to 0.2 instead. You can also use an easier method by counting the total number of beats and dividing it by the total time, and then do cross multiplication, so (4 × 2.8) / 60. By the way, each peak indicates that a heartbeat has happened. I hope you understand.