​If Today is 20^th November 2017 which is a Monday. What day will be on 20^th November 2020?
{Please find the answer in a mathematical order and manner. Please explain properly how to solve these questions which usually appears in olympiads}

Dear Student,

Please find below the solution to the asked query:

Given : ​If Today is 20^th November 2017 which is a Monday.

So,

Number of days after 20^th November 2017 in 2017  =  10 + 31 =  41

And we know 2018 and 2019 are not leap years , So

Number of days in 2018 =  365 

And

Number of days in 2019 = 365

But 2020 is a leap year , So

Number of days till 20^th November 2020 in 2020  = 31 + 29 + 31 + 30 + 31 + 30 + 31 +31 + 30 + 31 + 20 = 325 

Then,

Total days from 20^th November 2017 to 20^th November 2020 = 41 + 365 + 365 + 325 = 1096

We know number of days in a week = 7  and each day will repeat after a week , So

$\frac{1096}{7} = \mathbf{156}\frac{\mathbf{4}}{\mathbf{7}}$ , Here we get odd days  = 4

As 20^th November 2017 is a Monday. , So

1 odd days  =  Tuesday ,

2 odd days =  Wednusday ,

3 odd days = Thursday ,

4 odd days = Friday

Therefore,

On 20^th November 2020 we have Friday .                                                       ( Ans )

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