You are given the following information, but you may prefer to do some research for yourself.

1 Jan 1900 was a Monday. Thirty days has September, April, June and November. All the rest have thirty-one, Saving February alone, Which has twenty-eight, rain or shine. And on leap years, twenty-nine. A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.

How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?



In [13]:

    
require 'active_support/all'









    Out[13]:





false



In [21]:

    
date = DateTime.new(1901, 1, 1)

result = 0
while date < DateTime.new(2001, 01, 01)
  result += 1 if date.sunday?
  date = date + 1.months
end

result









    Out[21]:





171