In December, Sarah's website had 72,000 visitors. In December of the previous year it had 51,500 visitors.
Percentage increase =
* 100
Visitors in present year = 72000
Visitors in previous year = 51500
Increase in the number of visitors = 72000 - 51500 = 20500
Percentage increase =
* 100
=

= 39.80582 %
So the percent of increase in number of visitors = 39.8%