Solution:

This can be possible. They are not twins but triplets.

Solution:

He cannot marry his widow's sister in any country. This is because if his wife is a widow, he is already dead.

Solution:

Yes, you should switch the envelope. In the beginning when you picked up the envelope, you had a 1/3 probability of finding an offer letter in the envelope. There was 2/3 chance that the letter was there in the two envelopes on the table.

If you keep your selected envelope, you still have a 1/3 chance of finding an offer letter in that. However, since the interviewer has removed one empty envelope from the table, if you switch, you have a probability of 2/3 that the offer letter is inside that.

Solution:

Mayank took birth on Sunday.

Mayank was born 25 days before Rohit and thus the birth date of Mayank will be 8 Feb 1970.
Republic Day fell on Monday.
Number of days if counted from 26 January to 8 February will be 13.

After 13 days of Monday, it will be Sunday.

Solution:

If you see the picture, they formed a pentagonal shape and achieved the required.

Solution:

60 minutes

Explanation:
Roy will be able to consume all the five tablets in an hour.

Tablet 1 will be taken in 0 min.
Tablet 2 will be taken in 15 min.
Tablet 3 will be taken in 30 min.
Tablet 4 will be taken in 45 min.
Tablet 5 will be taken in 60 min.

Solution:

Yes, this can be possible if the match was between two female boxers.

Solution:

The sword could not have the words 'World War I' engraved on it. At that time, only one world war had taken place. No one knew that a second was to happen in future. Thus if it was engraved, the mention of the number was impossible.

Solution:

We believe you know that square is also a rectangle.

Solution:

There are six 'F' in this sentence. Are you astonished? Read the sentence again and this time, count them on your fingers carefully.