Gurpreet correctly remembers that his mother's birthday is before 23rd of April but after 19th of April, whereas his sister correctly remembers that their mother's birthday is not on or after 22nd of April. On which day in April is definitely their mother's birthday?
Twentieth or twenty-first.
According to Gurpreet = 20, 21, 22. According to his sister = not after 21st April. So, the birthday will be 20 or 21 April from both statements.