Discussion

Explanation:

Let us first calculate the day of the week on 1^st August 1980.

Here we need to break days till 1^st August 1980.

Complete years = 1979

Odd days in first 1600 years = 0
Odd days in next 300 years = 1
Odd days in next 79 years (19 leap and 60 non-leap years) = 19 × 2 + 60 × 1 = 98.

∴ Odd days in complete years = 0 + 1 + 98 = 99 odd days.

∴ Odd days in complete years = Remainder (99 ÷ 7) = 1 odd days.

Incomplete year (1980, a leap year)

Jan + Feb + Mar = 0 odd days,
Apr + May + Jun = 0 odd days,
Jul = 31
Aug = 1

∴ Odd days in incomplete year = 0 + 0 + 31 + 1 = 32 odd days = 4 odd days.

∴ Total odd days till 1^st August 1980 = 1 + 4 = 5 odd days.

∴ Day of the week on 1^st August 1980 will be 5 days after Sunday i.e., Friday.

Since 1^st August 1980 is Friday, next Monday will be on 4^th August.

Hence Mondays in August 1980 will be on 4^th, 11^th, 18^th and 25^th. 

Hence, option (a).

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