If it was Monday on 15th January 2010, then what was the day of the week on 15th January 2025? 

There are four leap years between 15 th January 2010 and 15 th January 2025 i.e., 2012, 2016, 2020, 2024. As, number of odd days in a normal year is '1' and number of odd days in a leap year is '2'. There are 11 normal years and 4 leap years. So, number of odd days from 15 th January 2010 and 15 th January 2025 is: (11 + 8) = 19 days The days of the week repeat itself after 7 days. Thus, the number of odd days = 5 As on 15 th January 2010, it was Monday, so, on 15 th January 2025 it will be Saturday