If Today Is Friday, what was the day before 21 days

How do we solve it ?

A week has 7 days " Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday ".

21 / 7 = 3 ( Remainder = 0)

So you can say that before multiples of 7 days, the day will repeat itself as friday.

Hence before 21 days Friday would occur again.

So we calculate before the remainder (0) , the answer is Friday.

Simple Logic = And whatever the remainder obtained is, deduct that to the day.