The heart beat is given in units of beats per minute, therefore before we can do any calculations let us make sure that all units are similar by converting the age in terms of minutes.
We know that there are 365 days per year, therefore:
90 years (365 days / year) = 32,850 days
We know that in a day, there are 24 hours, therefore:
32,850 days (24 hours / day) = 788,400 hours
And in an hour, there are 60 minutes, therefore:
788,400 hours (60 minutes / hour) = 47,304,000 minutes
Now calculating for the total heart beats in a lifetime:
total heartbeats = (80 beats / minute) * 47,304,000 minutes
total heartbeats = 3,784,320,000
Writing this into scientific notation:
3.78 x 10^9 beats
or
3.8 x 10^9 beats