If it was a Monday on 1 January 2018, what was the day of the week on 31 December 2020?

Starting date: 1 January 2018 is a Monday

Ending date: 31 December 2020

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To find the day of the week on 31 December 2020, calculate the total number of days from 1 January 2018 to 31 December 2020.

For 2018 (not a leap year): From 1 January to 31 December = 365 days

For 2019 (not a leap year): Full year = 365 days

For 2020 (leap year, divisible by 4): Full year = 366 days

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Total days from 1 January 2018 to 31 December 2018 = 365 - 1 = 364 days (excluding the start date)

Total days from 1 January 2018 to 31 December 2019 = 364 + 365 = 729 days

Total days from 1 January 2018 to 31 December 2020 = 729 + 366 = 1095 days

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To find the day of the week, divide the total days by 7:

$1095 \div 7 = 156$ remainder $3$

This means 31 December 2020 is 3 days after Monday.

Monday + 3 days = Thursday

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Therefore, 31 December 2020 was a Thursday.

The correct answer is Option 4: Thursday