A heart beats about 72 times a minute. Estimate how many times it beats in a life of 75 years, and…

Step 1. Minutes in a year. $60 \times 24 \times 365 = 525\,600 \approx 5.26 \times 10^{5}$.

Step 2. Minutes in a life. $75 \times 5.26 \times 10^{5} \approx 3.94 \times 10^{7}$.

Step 3. Beats. $72 \times 3.94 \times 10^{7} \approx 2.8 \times 10^{9}$.

Step 4. Order of magnitude. The front, $2.8$, is less than $5$, so it is $10^{9}$: about three billion beats.