Try this beautiful problem from TOMATO Objective no. 13 based on Calendar Problem. This problem is useful for BSc Maths and Stats Entrance Exams.
Problem:
June 10, 1979, was a SUNDAY. Then May 10, 1972, was a
(A) Wednesday;
(B) Friday;
(C) Sunday;
(D) Tuesday;
Solution:
In a (non-leap) year there are 365 days.
$365 \equiv 1 \mod 7 $
On a leap year, there are 366 days
$latex 366 \equiv 2 \mod 7 $
From 1972 to 1979, there are 7 years (1 of them is leap year). For each non-leap year, we have to go back 1 day and for every leap year, we have to go back 2 days. Hence in total, we have to go back 8 days for those 7 years. Also, May has 31 days. Hence we have to go back 31+8 = 39 days.
Thus $latex 39 \equiv 4 \mod 7 $ days before Sunday is a Wednesday.
Answer: (A) Wednesday.
