PPT Slide
Since 12 - 3 = 9, the Gregorian calendar eliminates 9 “century-leap-years” out of every 1,200 years. Thus, 300 - 9 = 291 years out of every 1,200 are leap years, and 900 + 9 = 909 are regular years.
Can you use this information to estimate the accuracy of the Gregorian calendar?