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smc

number theory senior

Problem

In year , the day of the year is a Tuesday. In year , the day is also a Tuesday. On what day of the week did the th day of year occur?
(A)
(B)
(C)
(D)
Solution
There are either or days between the first two dates depending upon whether or not year is a leap year (since the February 29th of the leap year would come between the 300th day of year and 200th day of year ). Since divides into but not , for both days to be a Tuesday, year must be a leap year. Hence, year is not a leap year, and so since there are days between the date in years , this leaves a remainder of upon division by . Since we are subtracting days, we count 5 days before Tuesday, which gives us
Final answer
A