This Instructional Activity is a part of the Lesson 6, Unit 9, Grade 7. This activity focuses on another complex calculation: the number of times our heart has beaten in our lifetime. In addition to all of the precision issues in determining how old we are from the previous activity, there is an additional level of complexity as our heart does not beat at a constant rate. For this situation, we need to make a reasoned estimate, but there is no hope of getting an exact answer. This makes our lack of knowledge of the exact time of our birth (or the exact time when we are solving the problem) unimportant because a few minutes or hours will not significantly impact the answer.